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A ZODB storage for replication using RAID techniques.

Project description

The ZEORaid storage is a storage intended to make ZEO installations more reliable by applying techniques as used in hard disk RAID solutions.

The implementation is intended to make use of as much existing infrastructure as possible and provide a seamless and simple experience on setting up a reliable ZEO server infrastructure.

Note: We use typical RAID terms to describe the behaviour of this system.

The RAID storage

The ZEORaid storage is a proxy storage that works like a RAID controller by creating a redundant array of ZEO servers. The redundancy is similar to RAID level 1.

Therefore, up to N-1 out of N ZEO servers can fail without interrupting the service.

It is intended that any storage can be used as a backend storage for a RAID storage, although typically a ClientStorage will be the direct backend.

The ZEORaid server

The RAID storage could (in theory) be used directly from a Zope server. However, to achieve real reliability, the RAID has to run as a storage for multiple Zope servers, like a normal ZEO setup does.

For this, we leverage the normal ZEO server implementation and simply use a RAID storage instead of a FileStorage. To achieve full reliability, you can install multiple ZEORaid servers with identical configuration:

[ Zope 1 ]                      [ ZEORaid 1 ]                  [ ZEO 1 ]
[ Zope 2 ]    talk to all -->   [ ZEORaid 2 ]   talk to all -> [ ZEO 2 ]
...                             ...                            ...
[ Zope N]                       [ ZEORaid N ]                  [ ZEO N ]

ZEO RAID servers maintain a list of all the optimal, degraded and recovering storages and provide an extended Storage API to allow querying the RAID status and disabling and recovering storages at runtime.


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