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From Apache Wiki <wikidi...@apache.org>
Subject [Couchdb Wiki] Update of "Frequently asked questions" by JanLehnardt
Date Mon, 02 Mar 2009 15:53:31 GMT
Dear Wiki user,

You have subscribed to a wiki page or wiki category on "Couchdb Wiki" for change notification.

The following page has been changed by JanLehnardt:
http://wiki.apache.org/couchdb/Frequently_asked_questions

The comment on the change is:
add transactions FaQ

------------------------------------------------------------------------------
    * [#how_replication How Do I Use Replication?]
    * [#how_find_conflicts How do I find out which conflicts occurred during replication]
    * [#how_spread_load How can I spread load across multiple nodes?]
-   * [#how_fast_views How Fast are CouchDB Views?]
    * [#why_no_mnesia Why Does CouchDB Not Use Mnesia?]
    * [#i_can_has_no_http Can I talk to CouchDB without going through the HTTP API?]
    * [#unicode_data Erlang has been slow to adopt Unicode. Is Unicode or UTF-8 a problem
with CouchDB?]
+   * [#transactions How do I use transactions with CouchDB?]
  
  === Views ===
+   * [#how_fast_views How Fast are CouchDB Views?]
    * [#update_views_more_often I want to update my view indexes more often than only when
a user reads it. How do I do that best?]
    * [#slow_view_building Creating my view index takes ages, WTF?]
    * [#relationships How do I model relationships in CouchDB? / Where are my JOINs?]
@@ -127, +128 @@

  == Can I talk to CouchDB without going through the HTTP API? ==
  
  CouchDB's data model and internal API map the REST/HTTP model so well that any other API
would basically reinvent some flavour of HTTP. However, there is a plan to refactor CouchDB's
internals so as to provide a documented Erlang API.
+ 
+ [[Anchor(transactions)]]
+ == How do I use transactions with CouchDB? ==
+ 
+ CouchDB uses an "optimistic concurrency" model. In the simplest terms, this just means that
you send a document version along with your update, and CouchDB rejects the change if the
current document version doesn't match what you've sent.
+ 
+ It's deceptively simple, really. You can reframe many normal transaction based scenarios
for CouchDB. You do need to sort of throw out your RDBMS domain knowledge when learning CouchDB,
though. It's helpful to approach problems from a higher level, rather than attempting to mold
Couch to a SQL based world.
+ 
+ Keeping track of inventory
+ 
+ The problem you outlined is primarily an inventory issue. If you have a document describing
an item, and it includes a field for "quantity available", you can handle concurrency issues
like this:
+ 
+ Retrieve the document, take note of the _rev property that CouchDB sends along
+ Decrement the quantity field, if it's greater than zero
+ Send the updated document back, using the _rev property
+ If the _rev matches the currently stored number, be done!
+ If there's a conflict (when _rev doesn't match), retrieve the newest document version
+ In this instance, there are two possible failure scenarios to think about. If the most recent
document version has a quantity of 0, you handle it just like you would in a RDBMS and alert
the user that they can't actually buy what they wanted to purchase. If the most recent document
version has a quantity greater than 0, you simply repeat the operation with the updated data,
and start back at the beginning. This forces you to do a bit more work than an RDBMS would,
and could get a little annoying if there are frequent, conflicting updates.
+ 
+ Now, the answer I just gave presupposes that you're going to do things in CouchDB in much
the same way that you would in an RDBMS. I might approach this problem a bit differently:
+ 
+ I'd start with a "master product" document that includes all the descriptor data (name,
picture, description, price, etc). Then I'd add an "inventory ticket" document for each specific
instance, with fields for product_key and claimed_by. If you're selling a model of hammer,
and have 20 of them to sell, you might have documents with keys like hammer-1, hammer-2, etc,
to represent each available hammer.
+ 
+ Then, I'd create a view that gives me a list of available hammers, with a reduce function
that lets me see a "total". These are completely off the cuff, but should give you an idea
of what a working view would look like.
+ 
+ Map
+ 
+ {{{
+ function(doc) 
+ { 
+     if (doc.type == 'inventory_ticket' && doc.claimed_by == null ) { 
+         emit(doc.product_key, { 'inventory_ticket' :doc.id, '_rev' : doc._rev }); 
+     } 
+ }
+ }}}
+ 
+ This gives me a list of available "tickets", by product key. I could grab a group of these
when someone wants to buy a hammer, then iterate through sending updates (using the id and
_rev) until I successfully claim one (previously claimed tickets will result in an update
error).
+ 
+ Reduce
+ 
+ {{{
+ function (keys, values, combine) {
+     return values.length;
+ }
+ }}}
+ This reduce function simply returns the total number of unclaimed inventory_ticket items,
so you can tell how many "hammers" are available for purchase.
+ 
+ Caveats
+ 
+ This solution represents roughly 3.5 minutes of total thinking for the particular problem
you've presented. There may be better ways of doing this! That said, it does substantially
reduce conflicting updates, and cuts down on the need to respond to a conflict with a new
update. Under this model, you won't have multiple users attempting to change data in primary
product entry. At the very worst, you'll have multiple users attempting to claim a single
ticket, and if you've grabbed several of those from your view, you simply move on to the next
ticket and try again
+ 
+ (This FaQ entry was borrowed from [http://stackoverflow.com/questions/299723/can-i-do-transactions-and-locks-in-couchdb]
with permission from the author.)
  
  [[Anchor(update_views_more_often)]]
  == I want to update my view indexes more often than only when a user reads it. How do I
do that best? ==

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