On Fri, Jun 24, 2016 at 04:44:02PM -0700, Ben Pfaff wrote:
> From: Mario Cabrera <[email protected]>
>
> The database replication functionality is designed to provide "fail
> over" characteristics. There are two participating databases, one of
> which is the "active" database and the other is the "stand by" database.
> Replication happens exclusively from the active to the stand by
> database.
>
> This document explains how the replication functionality is implemented.
>
> Signed-off-by: Mario Cabrera <[email protected]>
Thanks for the patch.
I noticed a few spelling errors and formatting that didn't quite match
what we usually do, so I'm folding in the following incremental.
diff --git a/Documentation/OVSDB-replication.md
b/Documentation/OVSDB-replication.md
index 4a4eb5e..74c9500 100644
--- a/Documentation/OVSDB-replication.md
+++ b/Documentation/OVSDB-replication.md
@@ -3,27 +3,32 @@ OVSDB replication implementation
Overview
========
-Given two Open vSwitch databases that have the same schema, OVSDB replication
-consists on maintaining these databases in the same state with one another,
-i.e each of the databases have the same contents at any given time even if they
-are not running in the same host. This document elaborates on the
implementation
-details to provide this functionality.
+
+Given two Open vSwitch databases with the same schema, OVSDB
+replication keeps these databases in the same state, i.e. each of the
+databases have the same contents at any given time even if they are
+not running in the same host. This document elaborates on the
+implementation details to provide this functionality.
Terminology
===========
-- Source of truth database: database whose content will be replicated to
-another database.
-- Active server: ovsdb-server providing RPC interface to the source of
-truth database.
-- Standby server: ovsdb-server providing RPC interface to the database that
-is not the source of truth.
+
+- Source of truth database: database whose content will be replicated
+ to another database.
+
+- Active server: ovsdb-server providing RPC interface to the source of
+ truth database.
+
+- Standby server: ovsdb-server providing RPC interface to the database
+ that is not the source of truth.
Design
======
-The overall design of replication consist on one ovsdb-server (active server)
+
+The overall design of replication consists of one ovsdb-server (active server)
communicating the state of its databases to another ovsdb-server
(standby server) so that the latter keep its own databases in that same state.
-In order to achieve this, the standby server acts as a client of the active
+To achieve this, the standby server acts as a client of the active
server, in the sense that it sends a monitor request to keep up to date with
the changes in the active server databases. When a notification from the
active server arrives, the standby server executes the necessary set of
@@ -43,6 +48,7 @@ databases. Below is the design represented as a diagram.
Setting up the replication
==========================
+
To initiate the replication process, the standby server must be executed
indicating the location of the active server via the command line option
"--sync-from=server", where server can take any form described in the
@@ -53,29 +59,37 @@ server responds.
When sending a monitor request the standby server is doing the following:
-1. Erase the content of the databases for which it is providing a RPC
interface.
+1. Erase the content of the databases for which it is providing a RPC
+interface.
+
2. Open the jsonrpc channel to communicate with the active server.
+
3. Fetch all the databases located in the active server.
-4. For each database with the same schema in both the active and standby
-servers: construct and send a monitor request message specifying the tables
-that will be monitored (i.e all the tables on the database except the ones
-blacklisted*).
-5. Set the standby database to the current state of the active database.
-
-Once the monitor request message is sent, the standby server will continuosly
-receive notifications of changes occuring to the tables specified in the
+
+4. For each database with the same schema in both the active and
+ standby servers: construct and send a monitor request message
+ specifying the tables that will be monitored (i.e all the tables on
+ the database except the ones blacklisted*).
+
+5. Set the standby database to the current state of the active
+ database.
+
+Once the monitor request message is sent, the standby server will continuously
+receive notifications of changes occurring to the tables specified in the
request. The process of handling this notifications is detailed in the next
section.
-*A set of tables that will be excluded from replication can be configure as a
-blacklist of tables via the command line option
"--sync-exclude-tables=db:table[,db:table]...",
-where db corresponds to the database where the table resides.
+*A set of tables that will be excluded from replication can be
+configure as a blacklist of tables via the command line option
+"--sync-exclude-tables=db:table[,db:table]...", where db corresponds
+to the database where the table resides.
Replication process
===================
-The replication proccess consists on handling the update notifications received
+
+The replication process consists on handling the update notifications received
in the standby server caused by the monitor request that was previously sent to
-the active server. In every loop interation, the standby server attempts to
+the active server. In every loop iteration, the standby server attempts to
receive a message from the active server which can be an error, an echo
message (used to keep the connection alive) or an update notification. In case
the message is a fatal error, the standby server will disconnect from the
@@ -84,40 +98,55 @@ standby server will reply with an echo message as well. If
the message is an
update notification, the following process occurs:
1. Create a new transaction.
+
2. Get the \<table-updates\> object from the "params" member of the
notification.
+
3. For each \<table-update\> in the \<table-updates\> object do:
+
1. For each \<row-update\> in \<table-update\> check what kind of
operation should be executed according to the following criteria about
the presence of the object members:
- - If "old" member is not present, execute an insert operation using
- \<row\> from the "new" member.
- - If "old" member is present and "new" member is not present, execute
- a delete operation using \<row\> from the "old" member
- - If both "old" and "new" members are present, execute an update
- operation using \<row\> from the "new" member.
+
+ - If "old" member is not present, execute an insert operation
+ using \<row\> from the "new" member.
+
+ - If "old" member is present and "new" member is not present,
+ execute a delete operation using \<row\> from the "old"
+ member
+
+ - If both "old" and "new" members are present, execute an
+ update operation using \<row\> from the "new" member.
+
4. Commit the transaction.
-If an error occurrs during the replication process, all replication is
+If an error occurs during the replication process, all replication is
restarted by resending a new monitor request as described in the section
"Setting up the replication".
+
Runtime management commands
===========================
+
Runtime management commands can be sent to a running standby server via
ovs-appctl in order to configure the replication functionality. The available
commands are the following.
- ovsdb-server/set-remote-ovsdb-server {server}: sets the name of the active
server.
+
- ovsdb-server/get-remote-ovsdb-server: gets the name of the active server
+
- ovsdb-server/connect-remote-ovsdb-server: causes the server to attempt to
send a monitor request every main loop iteration.
+
- ovsdb-server/disconnect-remote-ovsdb-server: closes the jsonrpc channel
between the active server and frees the memory used for the replication
configuration.
+
- ovsdb-server/set-sync-excluded-tables {db:table,...}: sets the tables list
that will be excluded from being replicated.
+
- ovsdb-server/get-sync-excluded-tables: gets the tables list that is
currently excluded from replication.
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