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Simple Indexer for Riak based on Links

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Link Indexing

This module allows you to create simple secondary indexes in Riak based on Riaks' link model. The basic idea is thus:

Assume we model person and companies as separate buckets:


When you store a /riak/person/Kresten object, you describe the employment relation by including this link in the Kresten object:

Link: </riak/company/Trifork>; riaktag="idx@employs"

The magic is that riak_link_index will then automatically add (and maintain) a link in the opposite direction; from Trifork to Kresten, and that link will have tag employs. The tag needs to start with idx@ for riak_link_index to recognize it.

Whenever you update or delete a person object, you can pass in new (or multiple) such links, and the old reverse links will automatically be deleted/updated as appropriate. Deleting a company object has no effect the other way around.

The objects that contain the reverse links (in this case e.g. /riak/company/Trifork) will have special content used to manage the links, so you cannot use them for other stuff!

This module also allows you to install an index_hook, which can be used to extract links from your objects. Index hooks can be written in both JavaScript for Erlang.


Notice! this only works on the master branch of Riak; this does not work on riak-0.14.* releases, because it depends on the pre- and post-commit hooks to both run in the same internal process (the riak_kv_put_fsm, if you must know).

To install, you need to make the ebin directory containing riak_link_index.beam accessible to your Riak install. You can do that by adding a line like this to riaks etc/vm.args

-pz /Path/to/riak_function_contrib/other/riak_link_index/ebin

If you're an Erlang wiz there are other ways, but that should work.

Next, you configure a bucket to support indexing. This involves two things:

  1. Install a set of commit hooks (indexing needs both a pre- and a post-commit hook).

  2. (optionally) configure a function to extract index information from your bucket data. We'll do that later, and start out with the easy version.

If your bucket is name person, it could be done thus:

prompt$ cat > bucket_props.json
{ "props" : {
  "precommit"  : [{"mod": "riak_link_index", "fun": "precommit"}],
  "postcommit" : [{"mod": "riak_link_index", "fun": "postcommit"}]
prompt$ curl -X PUT --data @bucket_props.json \
  -H 'Content-Type: application/json' \

There you go: you're ready for some action.

Explicit Indexing

The simple indexer now works for the person bucket, by interpreting links on /riak/person/XXX objects that have tags starting with idx@. The special idx@ prefix is recognized by the indexer, and it will create and maintain a link in the opposite direction, tagged with whatever comes after the idx@ prefix.

Let's say we add me:

curl -X PUT \
  -H 'Link: </riak/company/Trifork>; riaktag="idx@employs"' \
  -H 'Content-Type: application/json' \
  --data '{ "name": "Kresten Krab Thorup", "employer":"Trifork" }' \

As this gets written to Riak, the indexer will then create an object by the name of /riak/company/Trifork, which has a link pointing back to me:

curl -v -X GET
< 200 OK
< Link: </riak/person/Kresten>; riaktag="employs"
< Content-Length: 0

If there was already an object at /company/Trifork, then the indexer would leave the contents alone, but still add the reverse link. If no such object existed, then it would be created with empty contents.

Link Walking

The beauty of this is that you can now do link-walk queries to find your stuff. For instance, this link query should give you a list of person employed at Trifork. Lucky them :-)

curl http://localhost:8091/riak/company/Trifork/_,_,employs

Using a link_index hook

You can also install an index hook as a bucket property, which designates a function that can be used to decide which index records to create. This way you can keep the index creation on the server side; and also more easily generate some more indexes.

You install the index hook the same way you install a pre-commit hook; and the hook can be written in either Erlang or JavaScript, just like precommits.

// Return list of [Bucket,Key] that will link to me
function employmentIndexing(metaData, contents) {
  personData = JSON.parse(contents);
  if(personData.employer) {
    return [ ['company', personData.employer] ];
  } else {
    return [];

Assume you have that code in /tmp/js_source/my_indexer.s, and configured {js_source_dir, "/tmp/js_source"} in the riak_kv section of your etc/app.config.

Then, to install it as an indexer, you need to get install it as a bucket property in the person bucket. You can have more indexes, so it's a list of functions. Link-Index hooks can also be erlang functions.

prompt$ cat > bucket_props.json
{ "props" : {
  "link_index"  : [{"name": "employmentIndexing",
                    "tag" : "employs"}],
prompt$ curl -X PUT --data @bucket_props.json \
  -H 'Content-Type: application/json' \

Notice, that the link index also needs a tag property. You can install multiple index functions, but they should all have separate tags. Any idx@... tagged links that do not correspond to a registered link index are processed as "explicit indexing. In fact, the link_index hook is just a convenient way to have code insert the idx@-links for you.

Now, we can add objects to the person bucket without having to put the idx@employs link on the object. The index hook will do it for you. Happy you!

curl -X POST \
  -H 'Content-Type: application/json' \
  --data '{ "name": "Justin Sheehy", "employer":"Basho" }' \

While you can have multiple link_index'es, it is important that each link_index as its own distinguished tag, because riak_link_index will process each link index hook by first deleting any links with said tag, and then recomputing them based on the new content.


The indexer will handle delete/update of your records as appropriate, and should work fine with allow_mult buckets too. In fact, it is recommended to enable a allow_mult=true on the buckets containing the company objects (company in my example above), otherwise conflicting updates may be lost.

The indexer also manages conflicting updates to the link objects; which is pretty cool. Say, at the same time someone deletes some person object, and another process creates a new person object. In that case, the index object (in the company bucket) may end up with a conflicting update (i.e. get siblings); which would normally mean that someone has to take action on resolving the conflict. To manage this situation, riak_link_index stores a vclock-backed set in the content part of the index object (the company object), which is a set abstraction, which allows automatic merging based on each element in the set having its own vector clock. So, if someone adds a link, and someone else deletes a different link, then the result is quite easy to handle.

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