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2.9.8 Distributing load across several CVS servers

CVS can be configured to distribute usage across several CVS servers. This is accomplished by means of one or more write proxies, or secondary servers, for a single primary server.

When a CVS client accesses a secondary server and only sends read requests, then the secondary server handles the entire request. If the client sends any write requests, however, the secondary server asks the client to redirect its write request to the primary server, if the client supports redirect requests, and otherwise becomes a transparent proxy for the primary server, which actually handles the write request.

In this manner, any number of read-only secondary servers may be configured as write proxies for the primary server, effectively distributing the load from all read operations between the secondary servers and restricting the load on the primary server to write operations and pushing changes to the secondaries.

Primary servers will not automatically push changes to secondaries. This must be configured via `loginfo', `postadmin', `posttag', & `postwatch' scripts (see section The Trigger Scripts) like the following:

 
ALL	rsync -gopr -essh ./ secondary:/cvsroot/%p &

You would probably actually want to lock directories for write on the secondary and for read on the primary before running the `rsync' in the above example, but describing such a setup is beyond the scope of this document.

A secondary advantage of a write proxy setup is that users pointing at the secondary server can still execute fast read operations while on a network that connects to the primary over a slow link or even one where the link to the primary is periodically broken. Only write operations will require the network link to the primary.

To configure write proxies, the primary must be specified with the `PrimaryServer' option in `CVSROOT/config' (see section The CVSROOT/config configuration file). For the transparent proxy mode to work, all secondary servers must also be running the same version of the CVS server, or at least one that provides the same list of supported requests to the client as the primary server. This is not necessary for redirection.

Once a primary server is configured, secondary servers may be configured by:

  1. Duplicating the primary repository at the new location.
  2. Setting up the `loginfo', `postadmin', `posttag', and `postwatch' files on the primary to propagate writes to the new secondary.
  3. Configure remote access to the secondary(ies) as you would configure access to any other CVS server (see section Remote repositories).
  4. Ensuring that --allow-root=secondary-cvsroot is passed to all incovations of the secondary server if the path to the CVS repository directory is different on the two servers and you wish to support clients that do not handle the `Redirect' resopnse (CVS 1.12.9 and earlier clients do not handle the `Redirect' response).

    Please note, again, that writethrough proxy suport requires --allow-root=secondary-cvsroot to be specified for all incovations of the secondary server, not just `pserver' invocations. This may require a wrapper script for the CVS executable on your server machine.


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