Classifications

• • G—PHYSICS
  • G06—COMPUTING; CALCULATING OR COUNTING
  • G06F—ELECTRIC DIGITAL DATA PROCESSING
  • G06F16/00—Information retrieval; Database structures therefor; File system structures therefor
  • G06F16/20—Information retrieval; Database structures therefor; File system structures therefor of structured data, e.g. relational data
  • G06F16/27—Replication, distribution or synchronisation of data between databases or within a distributed database system; Distributed database system architectures therefor
  • G06F16/273—Asynchronous replication or reconciliation
• • G—PHYSICS
  • G06—COMPUTING; CALCULATING OR COUNTING
  • G06F—ELECTRIC DIGITAL DATA PROCESSING
  • G06F16/00—Information retrieval; Database structures therefor; File system structures therefor
  • G06F16/20—Information retrieval; Database structures therefor; File system structures therefor of structured data, e.g. relational data
  • G06F16/27—Replication, distribution or synchronisation of data between databases or within a distributed database system; Distributed database system architectures therefor

Definitions

• the present invention relates to a method of duplicating a database in a network of machines. It also relates to a duplicated database machine system. It applies in particular for the duplication of databases among a large number of machines connected in a network, for example in the context of air traffic management systems.
• Networked machine systems work with a growing number of connected machines. This is the case, for example, of air traffic management systems which may comprise several hundred machines, operating either as a server station or as a client station. In such a system, the machines access for example a common database.
• a base includes, for example, flight plan data, meteorological data, runway data, etc. ... All this data must be accessible by all machines in the network.
• the database is duplicated in each of these machines. Nevertheless, over time the data evolves. The database must therefore be duplicated regularly.
• the update of a data is performed by a machine of the network, in particular by a server.
• a first solution consists in transmitting point-to-point data from the station that is causing a data modification, for example a server, to the other stations of the network, for example client stations.
• a point-to-point transmission means that the transmitting station successively transmits the updated data to each client station.
• the overall update of the databases requires a very long time, not compatible with the current application.
• Another solution uses a multicast transmission.
• the multicast transmission makes it possible to carry out a single sending from the transmitting station, the updated data being simultaneously transmitted to all the other stations of the network, it must be ensured that the transmitted data are well received.
• it is necessary to use acknowledgments In this case, the transmitting station must wait for acknowledgments of all the machines in the network. Again, if the network has a large number of machines, waiting for all acknowledgments may take too long, incompatible application. To reduce this time, it may be possible to use not acknowledgments as indicated above but negative acknowledgments.
• negative acknowledgments means that a client machine, for example, re-acknowledges only if it does not receive data.
• a client machine for example, re-acknowledges only if it does not receive data.
• problems in detecting the non-receipt of data and the management of the software or hardware fault tolerance of the transmitter in particular In addition to these problems, there is still a wait time for negative acknowledgments to manage, this waiting time can also be too important.
• the subject of the invention is a method of duplicating a database among the machines of a network, a machine of the network transmitting an A data item updated in its database to the other machines. According to this method:
• the machine at the origin of the data A transmits towards all the machines in the same message with this data Has its version number and the address of the machine;
• the machine also transmits the message to a master machine
• a machine publishes the data item A in its database after a version number test; a master machine periodically transmits a central announcement message to all the machines, this central announcement message comprising only for each datum of the database its version number and the address of the machine at the origin of its update .
• a machine On receipt of a central announcement message (MAC), a machine compares the versions of the data of its base with the versions of the central announcement message (MAC), the machine requesting the transmission of a data item A to the master machine when the version number of the data A is lower than the version number of the central announcement message (MAC).
• MAC central announcement message
• the machine at the origin of the data A re-transmits for example this data to the other machines and the master machine when the version number of the data A that it has sent is greater than the version number of the announcement message.
• central (MAC) central
• the machines when a machine sends two linked data A, C, on reception the machines store the received data in a buffer zone, the linked data A, C being published in the local database only when they have the issued version by the original machine.
• the central announcement message (MAC) is issued periodically independent of the data transmission (A) by a machine (1).
• the machinery network may be an air traffic management system.
• the invention also relates to a system of networked machines sharing the same database, the base being duplicated in the machines, an original machine transmitting an A data update in its database to the other machines, the system comprising a master machine which converges the databases (10).
• a master machine which converges the databases (10).
• the machine (1) further transmitting the message to a master machine (31);
• the master machine (31) transmitting a central announcement message (32, MAC) to the machines (1, 2, 3), this central announcement message comprising for each datum of the base (10) its version number and the address of the machine (1) causing it to be updated; a machine (31, 2, 3) publishing the data item (A) in its database (10) after a version number test.
• a central announcement message 32, MAC
• the main advantages of the invention are that it applies to many systems sharing the same database, that it applies to systems with a very large number of machines without performance degradation, that it is simple to implement and that it provides a complete tolerance to multiple software or hardware failures.
• FIG. 1 an example of a network of machines sharing the same database, comprising, by way of example, a server and N client stations;
• FIG. 2 an exemplary structure of a database
• FIG. 3 an illustration of implementation of the method according to the invention applied to the network example of FIG. 1;
• FIG. 4 an exemplary structure of a central announcement message intended to converge the databases
• FIG. 5 an example of a central announcement message at the initialization of the databases
• FIG. 7 a state of the data table and the central announcement message associated with a given instant
• FIG. 8 an example of a structure of a message including the records transmitted by a machine of origin of the updates
• Figure 1 shows a network of machines. To simplify only three machines are represented, a server station 1 and two client stations 2, 3 of a network to N customers.
• the network could of course include other servers. Still for simplicity, only the server initiates the update of the data.
• the data could also be updated by other servers. They could also be updated by client computers.
• the machines 1, 2, 3 share the same database 10. This database updated by the server 1 is distributed on the clients 2, 3.
• FIG. 2 shows an example of a structure of such a database.
• the database 10 includes a data suite 21.
• each data corresponds to a record.
• This may be, for example, a flight plan record, weather data, runway data, or other information related to air traffic management.
• the database 10 therefore comprises a series of records 21 regularly updated by the server 1, these records being duplicated in each of the machines of the network. Subsequently, it will be considered by way of example that a datum 21 of the base 10 is a record. The latter could nevertheless include other types of data than records.
• FIG. 2 thus presents a base comprising P recordings 21. To describe an operation of the method according to the invention, the updates of two data, an A record and a C record, will be followed.
• the server writes a new record A and then by a multicast transmission 4, it transmits this record A to the client stations 2, 3.
• the multicast link allows the server to execute a single send, the record A being simultaneously distributed to all clients.
• the clients 2, 3 re-transmit to the server an accused positive reception or negative acknowledgment.
• a positive acknowledgment is sent to indicate that the record is received and a negative acknowledgment is sent to indicate that a record is not received. Disadvantages of these two modes of distribution have been mentioned previously.
• FIG. 3 illustrates an implementation of the method according to the invention.
• the networked system comprises the same machines 1, 2, 3 as the system of FIG. 1. It furthermore comprises a master machine 31.
• This machine can be for example a server or a client station. Preferably, this machine is dedicated to its master function as described below.
• the master hosts the database 10 As an example, we resume the update of the record A.
• the server therefore writes a new record A in its own database 10.
• the clients 2, 3 and the master 31 write the record A in the corresponding location 21 of the database 10.
• the clients 2, 3 and the master 31 do not send acknowledgments to the machine of origin of the record A, in this case the server 1 in the example of Figure 3.
• the method according to the invention periodically converts the data tables between them, independently of the updates.
• the period of convergence depends in particular on the application. It is the master 31 which contributes to ensuring the convergence of the data tables 10. For the proper functioning of the system it is especially necessary that at given times all the duplicate databases 10 comprise the same data. For example, all machines must have the same record of a flight plan at a given moment or the same meteorological information record to ensure the overall reliability of the system.
• the master station 31 periodically sends a central announcement message 32 (MAC) to all the other machines 1, 2, 3 of the network, including to the machine 1 at the origin of the updates. This MAC message is especially intended to converge the databases 10 of the machines 1, 2, 3 of the network.
• MAC central announcement message
• FIG. 4 shows the structure of a central announcement message 32 next to the database 10.
• This message actually comprises a series of data coupled to the records of the database 10.
• the base 10 includes a record, A for example.
• the base also comprises a counter 41 and an IP address 42.
• the counter indicates the version number of the record, more particularly the counter indicates the counting result. the number of changes made to the A record since a given initial record.
• This structure of a record A completed by its counter and the IP address of its original machine is repeated for all records in the database.
• a machine 1 which writes a new record A in a database increments the counter of this record and writes its own IP address to the reserved locations of the database. Then the machine 1 transmits the recording A, accompanied by its counter and its IP address to the other machines 2, 3 and in particular to the master station 31.
• the set of counters of all the records and the IP addresses of the original machines of the recordings forms the central announcement message 32 sent periodically by the master station 31 to all other machines.
• This message 32 is for example sent by the multicast link.
• the data of this message will be called later MAC data.
• the slaves On receipt of this MAC message sent by the master 31, the slaves perform a test with their own MAC data which are updated at the same time as the reception of the new recordings. These slaves being of course all the other machines 1, 2, 3 of the system that share the same database. If we take the case of the record A, when the server 1 writes and then transmits the new record A, it also transmits the updated counter and its IP address.
• Each client 2, 3 thus comprises the MAC data of a central announcement message 32. These data are updated as and when new records are received, line by line. Then periodically it receives from the master station the central announcement message 32 comprising all the MAC data. The client then compares its MAC data with the MAC data of the central announcement message 32 sent by the master 31.
• FIG. 5 presents a MAC message to the creation of a first record A.
• This first record is for example written by the server 1 in its database 10.
• the server also records the version number, that is to say say increments the counter of the record A to 1 and it registers its IP address in the corresponding field 42. Then the server transmits via the multicast link 4 the record A, the value of the counter, therefore the version number of the recording A, and its IP address to the other machines of the network, that is to say the client stations 2, 3 and the master 31.
• FIG. 6 presents the system of FIG. 3 after transmission of this first recording A.
• a client 3 did not receive the message.
• the counter remains at 0 at the location 41 reserved for the word A opposite the database 10, within this client 3.
• the other client 2 and the master having received the transmitted data by the server 1, have the correct version number of the record A, in this case version 1 here at initialization.
• the location 41 reserved for the counter of the record A is at 1 for this client 2 and for the master 31.
• the master sends the central announcement message 32, the MAC message.
• the MAC message has a known structure of all the machines, so that each of them can identify the counter associated with each of the records as well as the IP addresses of the transmitting machines.
• the MAC message has for example a serial structure which successively presents the counter and the IP address associated with each of the records.
• the MAC message is for example transmitted by the multicast link.
• the receiving machines 1, 2, 3 compare this MAC message transmitted by the master 31 with their own MAC message. In particular, they compare each of the version numbers of the words of the base 10 with the corresponding version numbers transmitted by the master. In the example of FIG.
• the machines compare the value of the counter of the slot 41 reserved for the word A with the value of the counter entered in the MAC message transmitted by the master. For the result of the comparison to be conclusive, the counter values must be identical, in this case equal to 1 in the example of FIG. 6. In this example, a client 3 has not received the registration. A and therefore its counter remained at 0 on the reserved slot 41. More generally, an A record was not received by a machine if:
• Cpt A m achine is the counter of the record A in the machine and Cpt Amaître is the counter of the record A in the MAC message sent by the master.
• the relation (1) does not apply, however, when the master 31 is faulty.
• the client station 3 realizes that the record A has not been transmitted following the test (1), it asks for example the issuance of this record A to the master station 31. More generally when a client station asks issuing records that do not have the same version number as the MAC message, that is, whose associated counters verify the relationship (1).
• the master 31 then retransmits the recordings requested by the client, for example in multicast mode. In multicast transmission, the record or records issued by the master 31 are sent to all other machines 1, 2, 3 including those that have received the registration. These other machines do the test according to relation (1) and do not react because this test then indicates that the records they contain have the correct version.
• the invention also advantageously makes it possible to treat the case of a machine that fits into the network.
• a machine requests from the master all the records of the database following the tests (1). Following the tests, the machine then acts as another machine with the wrong version numbers.
• the request for re-transmission of all the records by the master may result in the saturation of the network.
• the machine requests for example at first only part of the recordings. More generally, to avoid occasional overloads on the network, the master for example sends a subset of the records in a given period of time, or sends the same record once in a given period.
• the master 31 which does not receive the record A. Its counter then remains at zero, especially in the example of FIG. 6.
• the client 3 which is in the same state that the master, that is to say who has not received the record A does not react and therefore does not request the emission by the master of the recording A. Indeed, in this case the test according to relation (1) is inoperative. The client 3 does not check the relation (1) and yet it has not received the record A. This client station 3 does not notice that it did not receive this record A.
• a machine which verifies the relation (2) therefore knows that it has received the record A. It can therefore transmit to all the machines including the master the record A as well as all the other records for which it checks the relation (2). Nevertheless, to avoid saturation on the network, it is preferable that only one machine transmits the recording A. It can be then the machine which is to the origin of the modification of the record A, that is to say the writer. In the example of Figure 6 it is for example the server 1 to transmit the record A which it is the origin. The machine which is at the origin of a recording is well identified in the MAC message thanks in particular to its IP address placed at the location 42 opposite the counter.
• a client machine (among 1, 2, 3) is responsible for the retransmission of the recording A. This situation is detected by the machines that received record A if the MAC message issued by the master checks relation (2) for n consecutive periods.
• FIG. 6 shows a state of the table 10 and MAC message associated with a given time. This table 10 is duplicated in all the machines. In each machine the table 10 occupies a reserved memory space, a memory slot 21 is reserved for each record. It is the same for the MAC message that has a memory space.
• a series of memory cells 41 thus indicates the version numbers of the recordings, via counters.
• Another series of memory boxes 42 indicate the IP addresses of the original machines of the recordings. Records can be independent of each other, but they can also be linked. This is particularly the case when transactions are at stake. In practice there is a relationship between two tables and the updating of records must be done simultaneously. It is assumed by way of example that the data A and C are linked.
• the clients 2, 3 receive A and C at the same time. More specifically, it is necessary that in case of non-receipt of A by a customer, C is not visible by this customer. The client should avoid using a good version of C with a bad version of A.
• FIG. 8 shows an example structure of a message 81 comprising the records sent by the server 1.
• This message structure notably makes it possible to process the linked records.
• Message 81 has a serial data structure and has a header 82 followed by records.
• the header 82 indicates the number N of records contained in the message.
• the N records include the linked records A and C.
• a client will then have to place the received records in his database 10 only if he has received all the N records.
• a client knows that he has received a recording, in particular using the tests (1), (2) described above.
• Each client therefore has an internal buffer, also called buffer in the English literature, in which it stores the records received.
• FIG. 9 illustrates in particular the function of this buffer.
• a client 3 therefore comprises a buffer 91 in which it receives the linked records A, C and more generally all the other linked or independent records.
• the buffer occupies a dedicated memory area in the client.
• the client treats the stamp 91 as a record A, as described with reference to Figure 6 in particular.
• the method according to the invention applies to the buffer the protocol described above for an A record.
• the version of a buffer is considered correct when all the versions of its records A, C are correct.
• the client 3 asks the master 31 to re-transmit the records A, C.
• the client switches the data of the buffer into its buffer.
• a transmission fault affects the header 82, that is to say that the content thereof is not received by a client.
• the client then realizes that he has not received the contents of the header and sends a request to the master for a re-transmission of the complete message 81, including header.
• the master for example keeps in memory that the data A, C are grouped and then reissues the whole of the message 81 with the header 82.
• the convergence time in the event of breakdowns for an air traffic management application can be for example between one and five seconds.
• This convergence period corresponds to the period of sending the MAC messages, that is to say a MAC message is then sent every ⁇ t, ⁇ t being for example between 1 and 5 seconds.
• the sending of the records A, C updated by the multicast link 4 is done at any time, in particular regardless of the periods of convergence of the records.
• the master sends the MAC message and then starts the convergence transactions between the master and the clients: tests on the versions of the received recordings, request for re-transmission of the not received recordings, with or without iteration.
• all the databases 10 converge. They are identical, they have the same version.
• a method according to the invention does not manage records history, it only manages the latest version. In particular, if there are 10, 100 or even 1000 machines in a network, or even more, the performances are the same. The response times are not related to the numbers of the machines, in particular since the version tests (1), (2) are decentralized in each machine and the retransmissions after the re-transmission requests made in parallel are made by convergence period to all multicast machines.
• the version tests (1), (2) are decentralized in each machine and the retransmissions after the re-transmission requests made in parallel are made by convergence period to all multicast machines.
• FIGS. 3 and 6 only one server has been represented. An application can obviously use multiple servers. This is particularly the case for an air traffic management system where the system may include a server dedicated to flight plans, a server dedicated to meteorological data, a server dedicated to runway data, etc. ...
• each of these servers writes the corresponding records to the database 10.
• the server 1 and the master 31 are two separate machines. Nevertheless, it is possible to provide applications where the master 31 is also at the origin of the updating of the data. In other words, the same machine can be both master and writer, a writer being at the origin of the writing or the modification of a data or a recording.
• the invention has been described for an air traffic management system, but it can still apply for many other systems comprising networked machines that use the same database.

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• 2004
  • 2004-12-07 FR FR0413020A patent/FR2879056B1/fr not_active Expired - Fee Related
• 2005
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