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< p > Library Version 11.2.5.3< / p >
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Berkeley DB Replication
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< h2 class = "title" style = "clear: both" > < a id = "rep_mgrmulti" > < / a > Running Replication Manager in multiple processes< / h2 >
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< a href = "rep_mgrmulti.html#idp2239216" > One replication process and multiple subordinate processes< / a >
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< a href = "rep_mgrmulti.html#idp2202400" > Persistence of local site network address configuration< / a >
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< a href = "rep_mgrmulti.html#idp2221464" > Programming considerations< / a >
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< a href = "rep_mgrmulti.html#idp2233184" > Handling failure< / a >
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< a href = "rep_mgrmulti.html#idp2233360" > Other miscellaneous rules< / a >
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< p > Replication Manager supports shared access to a database environment
from multiple processes.< / p >
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< h3 class = "title" > < a id = "idp2239216" > < / a > One replication process and multiple subordinate processes< / h3 >
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< p > Each site in a replication group has just one network address (TCP/IP
host name and port number). This means that only one process can
accept incoming connections. At least one application process must
invoke the < a href = "../api_reference/C/repmgrstart.html" class = "olink" > DB_ENV-> repmgr_start()< / a > method to initiate communications
and management of the replication state.< / p >
< p > If it is convenient, multiple processes may issue calls to the
Replication Manager configuration methods, and multiple processes may
call < a href = "../api_reference/C/repmgrstart.html" class = "olink" > DB_ENV-> repmgr_start()< / a > . Replication Manager automatically opens
the TCP/IP listening socket in the first process to do so (we'll call
it the "replication process" here), and ignores this step in any
subsequent processes ("subordinate processes").< / p >
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< h3 class = "title" > < a id = "idp2202400" > < / a > Persistence of local site network address configuration< / h3 >
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< p > The local site network address is stored in shared memory,
and remains intact even when (all) processes close their environment
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handles gracefully and terminate. A process which opens an
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environment handle without running recovery automatically inherits the
existing local site network address configuration. Such a process may not
change the local site address (although it is allowed to redundantly
specify a local site configuration matching that which is already in
effect).< / p >
< p > In order to change the local site network address, the application
must run recovery. The application can then specify a new local site
address before restarting Replication Manager. The application should also
remove the old local site address from the replication group if it is no
longer needed.< / p >
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< h3 class = "title" > < a id = "idp2221464" > < / a > Programming considerations< / h3 >
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< p > Note that Replication Manager applications must follow all the usual
rules for Berkeley DB multi-threaded and/or multi-process
applications, such as ensuring that the recovery operation occurs
single-threaded, only once, before any other thread or processes
operate in the environment.
Since Replication Manager creates its own background threads which
operate on the environment,
all environment handles must be opened with the < a href = "../api_reference/C/dbopen.html#open_DB_THREAD" class = "olink" > DB_THREAD< / a > flag, even
if the application is otherwise single-threaded per process.< / p >
< p > At the replication master site, each Replication Manager process opens
outgoing TCP/IP connections to all clients in the replication group.
It uses these direct connections to send to clients any log records
resulting from update transactions that the process executes. But all
other replication activity —message processing, elections,
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etc.— takes place only in the "replication process".< / p >
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< p > Replication Manager notifies the application of certain events, using
the callback function configured with the < a href = "../api_reference/C/envevent_notify.html" class = "olink" > DB_ENV-> set_event_notify()< / a >
method. These notifications occur only in the process where the event
itself occurred. Generally this means that most notifications occur
only in the "replication process". Currently the only replication
notification that can occur in a "subordinate process" is
< a href = "../api_reference/C/envevent_notify.html#event_notify_DB_EVENT_REP_PERM_FAILED" class = "olink" > DB_EVENT_REP_PERM_FAILED< / a > .< / p >
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< p > It is not supported for a process running Replication Manager to
spawn a subprocess.< / p >
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< h3 class = "title" > < a id = "idp2233184" > < / a > Handling failure< / h3 >
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< p > Multi-process Replication Manager applications should handle failures
in a manner consistent with the rules described in
< a class = "xref" href = "transapp_fail.html" title = "Handling failure in Transactional Data Store applications" > Handling failure in Transactional Data Store applications< / a > .
To summarize, there are
two ways to handle failure of a process:< / p >
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The simple way is to kill all remaining processes, run
recovery, and then restart all processes from the beginning.
But this can be a bit drastic.
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Using the < a href = "../api_reference/C/envfailchk.html" class = "olink" > DB_ENV-> failchk()< / a > method, it is sometimes possible to
leave surviving processes running, and just restart the failed
process.
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Multi-process Replication Manager applications using this
technique must start a new process when an old process fails.
It is not possible for a "subordinate process" to take over the
duties of a failed "replication process". If the failed
process happens to be the replication process, then after a
failchk() call the next process to call < a href = "../api_reference/C/repmgrstart.html" class = "olink" > DB_ENV-> repmgr_start()< / a > will
become the new replication process.
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< h3 class = "title" > < a id = "idp2233360" > < / a > Other miscellaneous rules< / h3 >
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< li > A database environment may not be shared between a Replication
Manager application process and a Base API application process.< / li >
< li > It is not possible to run multiple Replication Manager processes
during mixed-version live upgrades from Berkeley DB versions prior
to 4.8.< / li >
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