History for ZEO_Replication

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**ZEO replication for school servers**

*I want school zope/CMF servers to always be available so that if one is down for any reason, another can take over, and when the original is back up, the second sends all the updates and both are synced again*

**Editorial** (Jan 25, 2002) ZEO (which used to cost $20,000 until released as open source) is a very elegant piece of work.  Yet it does not do everything and it took me a while to figure out the areas where it is useful (namely, for me: allowing several computers to share the execution load but share the ZODB, and as an environment for non-zope command line programming  on the ZODB). 

*Replication* and *External Mounting* products exist but again one must figure out the contexts where it makes sense to apply them.  I thought replication meant that two computers would continuously sync and serve the same database but with 2 copies instead of the default 1 copy in standard zeo.  That is currently not the case (and I don't know what the commercial version will do) ... it replicates for use in case the first server is out of service (which requires software to monitor and trigger these events).  External Mounting offers the hope of mounting several ZODB on the same zope service but without **undo** capability and possibly other interesting side effects.

Having taken a Reality Check, I now serenely respect ZEO and look forward to doing something similar to what that Brazilian Consultant wants to do with 2200 schools sharing ZODB with a ZEO web :-)  (see reference below)

**Issues**

- Must have same exact Zope installation, products, external methods on **each** Zope Client

- SQL access permission needed for all ZEO clients. Use ip address or hostname when you define the Z Database connection **not localhost**

- specify FORCE_PRODUCT_LOAD at one of your Zeo Clients, and new installed products will show up

- If using exUserFolder with postgresql source, that will not replicate by itself... which can be an advantage if authentication sources should vary from school to school

- Howto "handle multiple storages with zeo":http://lists.zope.org/pipermail/zodb-dev/2000-July/000350.html and "here:http://lists.zope.org/pipermail/zodb-dev/2000-July/000355.html

- **ZEO Advantage**: Command line scripts for crontab for example to "pack the database":http://www.zopelabs.com/cookbook/999418966

**References**

- "ZEO Product":http://www.zope.org/Products/ZEO and "Howto retrieve latest ZEO cvs version":http://www.zope.org/Members/jwb/zeo_from_cvs

- "Description of a large distributed ZEO environment by Michel":http://www.zope.org/Members/michel/MyWiki/ZEODescription

-  "Standalone ZODB":http://www.zope.org/Wikis/ZODB/StandaloneZODB allows lots of new possibilities with zeo

- "ZEO Howto load balance":http://www.zope.org/Members/kedai/UseZeoZope

- Chris Gray's "Howto setup ZEO on Debian":http://www.zope.org/Members/Chris%20Gray/DebianZEO

- "Zeo chapter from Zope Book":http://www.zope.org/Members/michel/ZB/ZEO.dtml

- "amk's HOWTO zodb and zeo (zope-less zeo)":http://www.amk.ca/zodb/zodb-zeo.html predicts creation of **whiteboard** with tkinter interface

- "Howo ZEO-Sessions":http://www.zope.org/Members/randy/ZEO-Sessions about using non-ZODB storage with zeo.  See also "Storage Benchmarks":http://www.zope.org/Members/htrd/benchmarks/storages

- "Mounted Storage Product":http://www.zope.org/Members/natsukashi/Products/MountedStorage
 based on older "External Mount":http://www.zope.org/Members/hathawsh/ExternalMount and "Mounted FIle Storage":http://www.zope.org/Members/anthony/software/MountedFileStorage

- "Howto share Products directory":http://www.zope.org/Members/Duncan/HowTos/ProductDirs *clever idea which might avoid having to replicate Products manually*

- "zeo/zodb development mailing list":http://lists.zope.org/pipermail/zodb-dev/ 

 - Thread about Brazilian Project for 2000 schools; 1 Zeo Server, 2000 Zeo Clients "posting":http://lists.zope.org/pipermail/zodb-dev/2001-December/002096.html suggestions to use XML-RPC for such large numbers, or modified ZEO, or regional zeo hubs

 - Replicated Storage (curently called **standby storage**) will be commercial $$$ with access to source ; ZEO 2 will work with that Replicated Storage while remaining open source.