CouchDB

CouchDB

1. API

   • Server
     • can serve as a sanity check to see if CouchDB is running at all. It can also act as a safety guard for libraries that require a certain version of CouchDB
     • Exmaple: {"couchdb":"Welcome","version":"0.10.1"}
   • Databases
     • a bucket that holds “related data.”
   • Documents
     •  central data structure
   • Replication
     • a mechanism to synchronize databases.
     • { "history": [ { "start_last_seq": 0, "missing_found": 2, "docs_read": 2, "end_last_seq": 5, "missing_checked": 2, "docs_written": 2, "doc_write_failures": 0, "end_time": "Sat, 11 Jul 2009 17:36:21 GMT", "start_time": "Sat, 11 Jul 2009 17:36:20 GMT" } ], "source_last_seq": 5, "session_id": "924e75e914392343de89c99d29d06671", "ok": true }

2. Design Documents

   • a CouchDB document with an id that begins with _design/
   • View Functions
     • Views are the primary tool used for querying and reporting on CouchDB databases.
     • Map Functions
       • accept a single document as the argument and (optionally) emit() key/value pairs that are stored in a view.
     • Reduce Functions
       • take two required arguments of keys and values lists - the result of the related map function - and an optional third value which indicates if rereduce mode is active or not. 
       • redfun(keys, values[, rereduce])
       • Built-Ins Funcs
         • _approx_count_distinct
           • Aproximates the number of distinct keys in a view index using a variant of the HyperLogLogalgorithm.
         • _count
           • Counts the number of values in the index with a given key.
   • Show Functions
     • are used to represent documents in various formats, commonly as HTML pages with nice formatting.
     • showfun(doc, req)
     • function(doc, req){ if (doc) { return "Hello from " + doc._id + "!"; } else { return "Hello, world!"; } }
     • function(doc, req){ return { 'json': { 'id': doc['_id'], 'rev': doc['_rev'] } } }
   • List Functions
     • While Show Functions are used to customize document presentation, List Functions are used for the same purpose, but on View Functions results.
     • The following list function formats the view and represents it as a very simple HTML page: function(head, req){ start({ 'headers': { 'Content-Type': 'text/html' } }); send('<html><body><table>'); send('<tr><th>ID</th><th>Key</th><th>Value</th></tr>') while(row = getRow()){ send(''.concat( '<tr>', '<td>' + toJSON(row.id) + '</td>', '<td>' + toJSON(row.key) + '</td>', '<td>' + toJSON(row.value) + '</td>', '</tr>' )); } send('</table></body></html>'); }
   • Update Functions
     • are functions that clients can request to invoke server-side logic that will create or update a document
     • function(doc, req){ if (!doc){ if ('id' in req && req['id']){ // create new document return [{'_id': req['id']}, 'New World'] } // change nothing in database return [null, 'Empty World'] } doc['world'] = 'hello'; doc['edited_by'] = req['userCtx']['name'] return [doc, 'Edited World!'] }
   • Filter Functions
     • Filter functions mostly act like Show Functions and List Functions: they format, or filter the changes feed.
     • special design document functions that allow the changes feed to emit only specific documents that pass filter rules.
     • Filter functions must return true if a document passed all the rules.
     • Classic
     • View
       • are the same as classic filters above, with one small difference: they use the mapinstead of the filter function of a view, to filter the changes feed.
       • Each time a key-value pair is emitted from the map function, a change is returned. This allows avoiding filter functions that mostly do the same work as views.
       • To use them just pass filter=_view and view=designdoc/viewname as request parameters to the changes feed GET /somedatabase/_changes?filter=_view&view=dname/viewname HTTP/1.1:
   • Validation Functions
     • can be used to prevent invalid or unauthorized document update requests from being stored. 
     • validatefun(newDoc, oldDoc, userCtx, secObj) newDoc – New version of document that will be stored. oldDoc – Previous version of document that is already stored.userCtx – User Context ObjectsecObj – Security Object

3. JSON Data Structures

   • Database Documents
     • total_rows
     • offset
     • update_seq
     • rows
   • CouchDB Document
   • Design Document
     • views
     • viewname
     • map
     • reduce
   • Request Object
     • body
     • cookie
     • form
     • headers
     • id
     • info
     • method
     • path
     • peer
     • query
     • requested_path
     • raw_path
     • secObj
     • uuid
   • Generic Fields
     • _id
     • _rev
     • userCtx
     • secObj