JPH1145195A - Computer system, abnormality detector and recording medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a computer system for grasping trouble, which occurs at an information managing server to be a managing object, without remote operation from the outside. SOLUTION: An information managing server 10 is provided with a system resource monitoring part 13 for preparing a notice message when the remaining storage capacitance of a DB 19 gets less than a reference capacitance, system error monitoring part 14 for monitoring a log and preparing a notice message when any abnormality is detected, and illegal invasion monitoring part 15 for comparing a process group under working with a process group at normal time and preparing a notice message when any abnormality is detected. These notice messages are automatically transmitted from the information managing server 10 to a system control managing server 30 by an electronic mail function. Thus, the trouble at the information managing server 10 can be easily grasped by the system control managing server 30.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a computer system comprising, for example, a plurality of computer devices connected to a wide area network. To improve the reliability of each computer device.

[0002]

2. Description of the Related Art In recent years, with the spread of large-scale and high-speed wide-area networks represented by the Internet and the like, computer systems for providing information to a plurality of users in various forms have been developed. In these computer systems, there is a demand for a highly reliable system construction and system operation management in consideration of a reduction in the efficiency of the entire system due to the occurrence of a failure, a security protection for an unauthorized intruder into the system, and the like. In general, a computer system constructed via a wide area network employs a client-server system type communication mode, in which a user accesses a server from a client-side computer device using an access interface. Then, desired electronic information is obtained. In such a computer system, for example, a WWW (World Wide Web) server, D
Servers responsible for specific processing, such as an NS (Domain Name Server) and a Proxy server, are distributed and arranged on a wide area network to provide information to users.

In each of these servers, in order to detect the cause of a system failure due to the occurrence of an abnormality, a system administrator or the like accesses the relevant server and performs various operations associated with the operation of an OS (operating system) and various applications. Generally, the contents of history information (hereinafter, log) such as an event log in which a system message is recorded at each time are verified, and a failure is dealt with after confirming whether there is any abnormality in the log. In addition, the necessary processes related to system operation are determined to some extent for each server. Therefore, by monitoring the disappearance of the processes and the occurrence of unnecessary processes, etc., it is possible to promptly deal with the detection of system failures or partial failures. ing.

In the above-described client-server system type server-side computer device, for example, an OS
Workstation with / UNIX installed (hereinafter referred to as UN
IX machine) and the like. This is UNI
X is TCP / IP (Transmission Control Protocol / In)
The main reason is that it provides an optimal environment for communication management based on the ternet Protocol).

[0005] On the other hand, a computer device employed on the client side is a low-cost PC compared to a UNIX machine.
(Personal computers) are the mainstream. However, in this PC, for example, Microsoft's Windows
OS with enhanced communication management as seen in wsNT, etc.
With the advent of the computer, it has been adopted as a server-side computer device.

In this case, for example, a server that manages communication with a plurality of relatively small clients is constructed by a PC. Hereinafter, a server constructed by a PC is referred to as a PC server. In addition, a similar PC server is constructed for each of a plurality of clients, and an integrated management (NOC) (NetworkOpe) for these PC servers is performed.
rations Center), UNIX server
It is built on a machine, etc., to manage and operate the entire system. Also, if a system failure or the like occurs in the PC server,
It is configured to perform remote access from the NOC to the corresponding PC server to cope with the failure.

[0007]

However, when a computer system is constructed using a PC server and a center server as described above, there are the following problems. (1-1) The acquisition and inspection of the log of the PC server by the center server are performed by remote access to the corresponding PC server via a public network, so that the operation is generally complicated. (1-2) In the center server or the PC server, a list of running processes can be obtained, but the necessity / unnecessity of those processes must be determined by a human. (1-3) The center server needs to perform regular maintenance to cope with the occurrence of a system abnormality, a failure, or the like in the PC server. (1-4) When the operation of the PC server is adversely affected due to a decrease in the capacity of a storage device such as a disk provided or connected to the PC server, the cause of the adverse effect cannot be detected. (1-5) For example, when a weak point of security (security hole) in the system is discovered by an intruder via a public network and illegally invaded, the intrusion can be detected from the state of the process. is there. This is due to the fact that a new process always occurs when an intruder starts an application in the system, but unnecessary processes occur in the PC server,
It cannot detect the loss of necessary processes. (1-6) In the center server, when investigating the cause of the system failure that makes the PC server unable to start again, it is not possible to acquire (understand) the circumstances leading up to the failure.

On the other hand, there is also the following problem depending on the PC. (2-1) For example, in a PC environment such as Windows NT, an e-mail system having higher versatility than a UNIX machine is not provided. (2-2) In the Windows NT PC environment, the S-JIS code is used for the kanji code.
The code cannot use SMTP (Simple Mail Transfer Protocol), which is an electronic mail exchange protocol of TCP / IP. (2-3) D widely used in Microsoft OS
The LL (Dynamic Link Library) is configured to be combined with an application when a program is executed and to be unloaded when the program is not needed. However, in the IIS (Internet Information Server) in the WWW environment, a server is used. DL that will be a security hole
Unnecessary change of L path cannot be monitored. (2-4) The center server cannot dynamically control parameter change processing for setting the operation of the PC server in a remote environment via a public network.

Therefore, an object of the present invention arises in a computer system including a managed computer device such as a PC server or a center server and a managed computer device for remotely monitoring the computer device. An object of the present invention is to make it possible to easily ascertain an existing defect without remote control from the management computer device side. Specifically, when a decrease in the storage capacity or a malfunction in the operating status occurs in the managed computer device, the managed computer device is immediately notified of the occurrence, and a serious abnormality is detected in the managed computer device. In this case, the management computer device grasps the situation and analyzes the process of the failure of the management target computer device. Another object of the present invention is to provide an abnormality detection device suitable for implementing the computer system, and a recording medium for realizing the abnormality detection device with a general-purpose computer device.

[0010]

According to another aspect of the present invention, there is provided a computer system comprising: a first computer device; and a second computer device connected to the first computer device via a communication line. In a computer system in which a first computer device is remotely monitored by a device, the first computer device periodically monitors whether there is an abnormality in its own device, and a device that creates an e-mail addressed to the second computer device. Each time the monitoring means detects an abnormality, an electronic mail indicating information on the abnormality is created and notified to the second computer device.

The monitoring means monitors, for example, (1-1) the remaining capacity of the storage device accessed by the own apparatus, and detects an abnormal state when the remaining capacity becomes equal to or less than a preset reference amount. Resource monitoring means for creating information, (1-
2) History information monitoring means for monitoring a change state of the history information indicating the operation status of the own apparatus and generating detection information indicating an abnormal state when the change level exceeds the notification level set in advance. (1-3) Specifying a process group in operation in its own device based on preset required process group information and unnecessary process group information, and specifying the specified process group with the required process group information and unnecessary process group information. A process monitoring means for generating detection information indicating an abnormal state when there is a process not included in or included in the process; (1-4) identification information preliminarily holding an identifier of electronic information stored in the storage device; And periodically activates at least one of identification information monitoring means for creating detection information indicating an abnormal state when the identifier does not match the identification information. Configured to create the e-mail when the respective detection information is output.

The electronic mail is created by converting the detection information into a predetermined SMTP compatible code.

According to another aspect of the present invention, there is provided an abnormality detecting apparatus for monitoring the remaining capacity of a storage device accessed by the apparatus, and the remaining capacity being equal to or less than a predetermined reference amount. A resource monitoring means for generating detection information indicating an abnormal state in the case; (2-2) monitoring a change state of the history information indicating an operation state of the own apparatus, and generating a history information having a change level exceeding a preset level; History information monitoring means for creating detection information indicating an abnormal state when detected, (2-3)
The running process group in the own device is specified based on the preset necessary process group information and the unnecessary process group information, and the running process group is included in the necessary process group information and the unnecessary process group information. (2-4) a process monitoring unit that creates detection information indicating an abnormal state when there is a missing process or a lacking process, (2-4) comparing the identifier of the electronic information stored in the storage device with the identification information held in advance, Identification information monitoring means for creating detection information indicating an abnormal state when the identifier does not match the identification information;
And (2-5) means for creating an electronic mail in a predetermined format when any of the detection information is output and presenting the generated electronic mail to an external device.

The history information monitoring means holds history information acquired at the time of initial operation as initial history information, for example.
When the history information after the next time is updated, the history information based on the difference between the initial history information and the updated history information is held as new initial history information.

According to another aspect of the present invention, there is provided a recording medium storing a program readable by a computer having a storage device, wherein the program includes any one of the following processes: Is periodically and repeatedly executed by the computer device. (3-1) processing for monitoring the remaining capacity of the storage device and creating detection information indicating an abnormal state when the remaining capacity is equal to or less than a preset reference amount;
2) a process of monitoring a change state of the history information indicating the operation state of the apparatus and creating detection information indicating an abnormal state when detecting a change level of the change exceeds a preset level; (3-3) Identify the running process group based on the preset necessary process group information and the unnecessary process group information, and specify the processes that are not included in the necessary process group information and the unnecessary process group information in the operating process group or (3-4) comparing the identifier of the electronic information stored in the storage device with the identification information stored in advance, and determining the identifier as the identification information. The process of creating detection information that indicates an abnormal state when they do not match.

[0016]

Embodiments of the present invention will be described below in detail with reference to the drawings. (First Embodiment) FIG. 1 is a functional block diagram showing an embodiment in which the present invention is applied to a computer system for providing information. This computer system 1
An information management server 10, a plurality of clients 20, and a system management server 3 for managing the entire system
0, and the information management server 10 and the system overall management server 30 are connected via a public network L so as to be capable of two-way communication.

The information management server 10 is realized by a general-purpose computer device and a computer program read and executed by the computer device.
The OS of the computer device is a multitasking type OS, for example, Windows NT. This information management server 1
0 is a database (DB) 19, a remote management unit 11, WWW, e-mail, Proxy, DN formed by executing the computer program.
It has the functions of a server function management unit 41 that performs processing such as S, and a communication management unit 42. The remote management unit 11 includes a remote monitoring unit 11 including a system resource monitoring unit 13, a system error monitoring unit 14, and an unauthorized intrusion monitoring unit 15, a message creation unit 16, a code conversion unit 17, and an abnormality notification unit 18. It is comprised including.

Note that the computer program is:
Usually, the computer device is stored in an internal or external storage device, read and executed at any time, but a recording medium that can be separated from the computer device,
For example, it may be stored in a CD-ROM, FD, or the like, installed in the internal storage device or the external storage device at the time of use, and provided for execution at any time.

The DB 19 stores a plurality of pieces of electronic information used in the server function management unit 41, logs in the information management server 10, information on a reference amount used for comparing the remaining capacity of the storage capacity, and information on normal operation of the information management server 10. It stores process group list information, process group list information that can be ignored when the information management server 10 is operating, identification information of the electronic information, and the like. The remote management unit 11 (remote monitoring unit 12), server function Management unit 41
Are configured to be able to exchange data with each other.

The remote management unit 11 includes a remote monitoring unit 1
The status information of the operation and operation of the information management server 10 acquired in step 2, the creation of a message corresponding to the status information, and the notification of the message are performed. This message is transmitted via the server function management unit 41 and the communication management unit 42, and further through the public network L.
Sent to 0.

The remote monitoring unit 12 is an information management server 1
Monitoring processing of the storage capacity of the DB 19, which is a resource at 0, the log relating to the operation of the information management server 10, and the operating state of the information management server 10 is performed. In this monitoring process,
Monitoring information is periodically acquired by a system resource monitoring unit 13, a system error monitoring unit 14, and an unauthorized intrusion monitoring unit 15, which will be described later. These pieces of monitoring information are input to the message creation unit 16, where a corresponding message is created.

In the remote monitoring unit 12, the system resource monitoring unit 13 monitors the storage capacity of the DB 19 and other storage devices provided in the information management server 10, and the system error monitoring unit 14 The management server 10 monitors the system log, the application log, and the security log. Here, the “system log” refers to the information management server 10
The “application log” is a log related to an application running on the information management server 10 and is a “security log”.
Is a log relating to information such as the identification of a user who has access to the information management server 10 from the client 20 or the system general management server 30, or authority. When the log changes, each of the monitoring units 13 to 15 creates monitoring information corresponding to a difference in the change that exceeds a preset notification level. The notification level is
For example, for the three types of logs described above, the type is set in advance such as notification of only an error or notification of a warning and an error for each type.

The unauthorized intrusion monitoring unit 15 includes the information management server 1
0, when there is an unauthorized intruder via the public network L, the intrusion is monitored by process management. This monitoring process is specifically performed by D
Based on the normally operating process group list information and the ignorable process group list information in B19, the process group actually running in the information management server 10 is compared with the process group list information, and the comparison is performed. If there is a difference in information, the corresponding monitoring information is created.

The negligible process group list information describes a type of process that terminates its own operation as needed without being resident. The intrusion monitoring unit 15 uses this type of process. Is not targeted for the creation of monitoring information.

Further, the unauthorized intrusion monitoring unit 15 monitors an unauthorized change to the identification information of the electronic information stored in the DB 19. In this monitoring process, the identification information described correctly is held, and when the identification information of the electronic information is illegally changed in the information management server 10, corresponding monitoring information is created. More specifically, a URL (Uniform Resour
ce Locator) Holds a normal value of the path name of the filter DLL such as a path name, compares the normal value with a changed value when the path name of the filter DLL is changed, and obtains a path name of the filter DLL. If it is determined that has been changed, corresponding monitoring information is created.

The message creating unit 16 creates a message to the system management server 30 corresponding to the monitoring information input in each of the functional blocks 13 to 15 of the remote monitoring unit 12. The created message is the code converter 1
7 is input.

The code conversion unit 17 includes the message creation unit 1
6 is converted to a message of a code corresponding to SMTP. For example, Wi
In the case of the Windows NT, the kanji code used is "S-JIS", so that the kanji code is converted to JIS7 for SMTP. The code-converted message is input to the abnormality notification unit 18.

The abnormality notifying unit 18 inputs the message converted by the code converting unit 17 to the server function managing unit 41. The input message is transmitted to the system management server 30 via the communication management unit 42 by using the e-mail function in the server function management unit 41.

Next, a detailed operation of the information management server 10 according to the present embodiment will be described. FIG. 2 is a processing procedure diagram of the system resource monitoring unit 13. The system resource monitoring unit 13
The monitoring information is created by checking the remaining storage capacity in the DB 19 provided in the information management server 10 (step S101). When the remaining capacity is larger than the reference amount of the reference amount information preset in the DB 19 (step S1)
02: No), the system resource monitoring unit 13 continues to monitor the storage capacity at preset time intervals (step S1).
03). On the other hand, when the remaining capacity is smaller than the reference amount (Step S102: Yes), the message creating unit 16 creates a message to be notified to the system management server 30 based on the monitoring information created in Step S101 (Step S102). Step S104). The message in this case is, for example, time information determined to be below the reference amount,
It is created including remaining capacity information, a drive name corresponding to the capacity, and the like.

The created message is transmitted to the code conversion unit 1
7, the code is converted into an SMTP-compatible code (step S105).
1 through the communication management unit 42 using the e-mail function,
A message corresponding to SMTP is transmitted (step S1).
06). After the end of the process, the system resource monitoring unit 13
The monitoring of the storage capacity is continued at preset time intervals (step S107).

FIG. 3 is a processing procedure diagram of the system error monitoring unit 14. The system error monitoring unit 14 checks all logs accumulated in the DB 19 provided in the information management server 10 (Step S201). If a changed log is not detected as compared with the previous log (step S202: No), the system error monitoring unit 14
Continues monitoring the log at a preset time interval (step S203). If the changed log is detected (step S202: Yes), monitoring information is created from the difference information between the log and the previous log, and the log is assigned to any of “system”, “application”, and “security”. (Step S20)
4). In the monitoring information created in step S204, it is checked whether there is a log exceeding the notification level set in advance (step S205), and the log exceeding the notification level is selected.
A message corresponding to the corresponding log for notifying the system management server 30 is created (step S20).
6).

The created message is stored in the above step S
Steps S207 to S207 are the same as steps 105 to 106.
8 and a message corresponding to the SMTP is transmitted, and the log monitoring information corresponding to the message is stored (step S209). By performing the process of storing the monitoring information, the information recorded in the log comes to be composed of new information. After the process is completed, the system error monitoring unit 14 continues to monitor the log at a preset time interval (step S210).

FIG. 4 is a processing procedure diagram of the unauthorized intrusion monitoring unit 15. The intrusion monitoring unit 15 checks the operation mode. If the operation mode is the process monitoring mode (step S
301: process), and checks a group of processes running on the information management server 10 (step S302). When there is a process corresponding to the preset ignorable process group list in the process group (step S3).
03: Yes), ignore the process (step S3)
04). If there is no difference between the normal process group list and the checked process group (Step S)
(305: No), the unauthorized intrusion monitoring unit 15 continues monitoring the log at a preset time interval (step S30).
6). If there is a difference (step S305, Yes), monitor information is created from the difference information, and the message creation unit 16
Creates a message corresponding to the monitoring information to be notified to the system management server 30 (step S3).
07). The created message is transmitted in step S10
Steps S308 to S309 are performed in steps S308 to S309 to transmit an SMTP-compatible message. After the processing is completed, the intrusion monitoring unit 15 continues to monitor the log at a preset time interval. Yes (Step S3
10).

On the other hand, if the operation mode is the electronic information identification information change monitoring mode (step S301: identification information), the path name of the current filter DLL is checked (step S311). If the path name of the DLL is the same as the normal path name of the preset path name of the filter DLL (step S312, Yes), the intrusion monitoring unit 1
No. 5 continues monitoring the log at a preset time interval (step S313). If they are not the same (step S3
12, No), the same processing as in steps S307 to S309 is performed. After the processing is completed, the intrusion monitoring unit 15
Continues monitoring the change of the identification information at a preset time interval (step S318).

In the information management server 10, for example,
By using the registry function of WindowsNT to set system parameters such as monitoring time intervals in the remote monitoring unit 12, the setting parameters are dynamically updated from the system general management server 30 to the information management server 10. ing.

As described above, in the computer system 1 according to the present embodiment, the information management server 10 performs process management by previously providing a process group list that operates normally and a process group list that can be ignored even if it operates. Therefore, the necessity / unnecessity of the process is automatically determined. Further, since the storage capacity of a disk or the like accessed by the information management server 10 is compared with a preset reference amount and the actual remaining storage capacity, a cause of an operation failure due to a decrease in the capacity can be detected. In addition, since process management can detect the occurrence of unnecessary processes and the disappearance of necessary processes, etc.
The intrusion can be detected from the state of the process.

The information management server 10 converts the kanji code of the message to be notified to the system general management server 30 from the S-JIS code to JIS7 and transmits the e-mail by the code conversion corresponding to SMTP.
In the PC environment of WindowsNT, compatibility with other e-mail systems can be provided, and the normal value of the path name of the filter DLL is stored in advance and the DLL
By detecting and notifying that the path name has been changed, it is possible to monitor for an unauthorized DLL path change. In addition, the information management server 10 employs a Windows NT registry function for setting system parameters, so that the system general management server 30 can perform setting parameter change processing and the like to the information management server 10 through a remote environment via a public network. And dynamic control becomes possible.

Further, the information management server 10 notifies the system management server 30 of information on the log of the own site and information on abnormalities and failures. Without remote access, log acquisition and inspection can be performed, periodic management is not required, and a serious system failure has occurred such that the information management server 10 cannot be started again. Even in this case, it is possible to obtain (understand) the process leading to the obstacle.

(Second Embodiment) The present invention can be implemented as an abnormality detection device using a stand-alone computer device. The abnormality detection device in this case is
It has a DB 19 built in a stand-alone computer device or in an external storage device, and has the same functional blocks as the information management server 10 of the computer system 1. Intrusion monitoring unit 15, message creation unit 16, code conversion unit 17, and abnormality notification unit 18,
It comprises.

This anomaly detection device is different from the information management server 10 in that a display device for presenting information to the anomaly detection device is provided, and that the remote management unit 11, the remote monitoring unit 12, and the communication management unit 42 The functional block is not provided, and the process corresponding to the communication management unit 42 is to configure the abnormality notification unit 18 to output a message to an output device such as a display device included in the abnormality detection device. Can be substituted. In this abnormality detection device, the same effect as the information management server 10 can be obtained by monitoring the storage capacity, the log, and the process.

[0041]

As is clear from the above description, according to the present invention, by monitoring the operation and operation of the information management server, it is possible to prevent in advance the malfunction of the operation status due to the decrease in the storage capacity of the disk or the like. In addition to preventing failures, when an error is detected, the information is notified to an external information management server using e-mail, so that even if a specific information management server completely fails, the history of the failure can be logged. As a result, the reliability and operation management of the entire system are significantly improved.

[Brief description of the drawings]

FIG. 1 is a functional block diagram illustrating an embodiment of a computer system according to the present invention.

FIG. 2 is a processing procedure diagram in a system resource monitoring unit.

FIG. 3 is a processing procedure diagram in a system error monitoring unit.

FIG. 4 is a processing procedure diagram in an unauthorized intrusion monitoring unit.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Computer system 10 Information management server 11 Remote management part 12 Remote monitoring part 13 System resource monitoring part 14 System error monitoring part 15 Intrusion monitoring part 16 Message creation part 17 Code conversion part 18 Abnormality notification part 19 Database (DB) 20 Client 30 System management server 41 Server function management unit 42 Communication management unit

Claims (6)

[Claims] 1. A computer system comprising: a first computer device; and a second computer device connected to the first computer device via a communication line, wherein the second computer device remotely monitors the first computer device. The first computer device includes: a monitoring unit that periodically monitors the presence or absence of an abnormality in its own device; and a unit that creates an e-mail addressed to the second computer device. Each time the monitoring unit detects the abnormality, A computer system, wherein an electronic mail representing information on an abnormality is created and notified to the second computer device. 2. A resource for monitoring the remaining capacity of a storage device accessed by its own device and generating detection information indicating an abnormal state when the remaining capacity becomes equal to or less than a predetermined reference amount. Monitoring means for monitoring the change state of the history information indicating the operation status of the own device, and creating detection information indicating an abnormal state when detecting the history information whose change level exceeds a preset notification level; Means, a process group in operation in the own device is specified based on preset required process group information and unnecessary process group information, and the specified process group is included in the required process group information and the unnecessary process group information. Process monitoring means for creating detection information indicating an abnormal state when there is no process or missing process; identifier of electronic information stored in the storage device In comparison with previously held identification information, at least one of the identification information monitoring means for creating detection information indicating an abnormal state when the identifier does not match the identification information is periodically activated, and the detection information is 2. The computer system according to claim 1, wherein the electronic mail is created when the electronic mail is created. 3. The computer system according to claim 2, wherein the monitoring unit converts the created detection information into a predetermined SMTP compatible code to create the electronic mail. 4. A resource monitoring means for monitoring the remaining capacity of a storage device accessed by its own device and creating detection information indicating an abnormal state when the remaining capacity becomes equal to or less than a predetermined reference amount. History information monitoring means for monitoring a change state of the history information indicating the operation status and generating detection information indicating an abnormal state when detecting the change information whose change level exceeds a preset level; Are identified based on preset required process group information and unnecessary process group information, and processes that are not included in the required process group information and unnecessary process group information or are missing in the operating process group. A process monitoring unit that creates detection information indicating an abnormal state when there is a process; and an identifier of electronic information stored in the storage device is held in advance. An identification information monitoring unit that creates detection information indicating an abnormal state when the identifier does not match the identification information, and further includes a predetermined information when any of the detection information is output. Means for creating an electronic mail in a format and presenting it to an external device. 5. The history information monitoring means holds history information acquired at the time of initial operation as initial history information, and when history information subsequent to the next time is updated, the initial history information and the updated history information are updated. 5. The abnormality detection device according to claim 4, wherein the history information based on a difference between the abnormality detection information and the difference information is held as new initial history information. 6. A storage medium storing a program readable by a computer device having a storage device, wherein the program monitors a remaining capacity of the storage device, and the remaining capacity is a predetermined reference. The process of creating detection information indicating an abnormal state when the amount becomes less than the amount, monitoring the change state of the history information indicating the operation status of the device, and detecting the history information whose change level exceeds a preset level. A process of creating detection information indicating an abnormal state; identifying a running process group based on preset required process group information and unnecessary process group information; and determining the required process group information and A process of creating detection information indicating an abnormal state when there is a process that is not included in the unnecessary process group information or a process that is insufficient, and the storage Comparing the identifier of the electronic information stored in the storage device with the identification information stored in advance, and generating detection information indicating an abnormal state when the identifier does not match the identification information. A recording medium characterized by being repeatedly executed periodically.