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https://issues.apache.org/jira/browse/AVRO-1124?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14173844#comment-14173844
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Sam Pullara commented on AVRO-1124:
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You need a "custom service" because the client may not have the schema.
Sending the version is great if your client happens to have copies of all
of them. Without a service to vend those schemas though you can't send a
message to another service that doesn't have the latest ones yet.
On Thu, Oct 16, 2014 at 7:41 AM, Zoltan Farkas (JIRA) <j...@apache.org>
> RESTful service for holding schemas
> -----------------------------------
>
> Key: AVRO-1124
> URL: https://issues.apache.org/jira/browse/AVRO-1124
> Project: Avro
> Issue Type: New Feature
> Reporter: Jay Kreps
> Assignee: Jay Kreps
> Attachments: AVRO-1124-can-read-with.patch, AVRO-1124-draft.patch,
> AVRO-1124-validators-preliminary.patch, AVRO-1124.2.patch, AVRO-1124.3.patch,
> AVRO-1124.4.patch, AVRO-1124.patch, AVRO-1124.patch
>
>
> Motivation: It is nice to be able to pass around data in serialized form but
> still know the exact schema that was used to serialize it. The overhead of
> storing the schema with each record is too high unless the individual records
> are very large. There are workarounds for some common cases: in the case of
> files a schema can be stored once with a file of many records amortizing the
> per-record cost, and in the case of RPC the schema can be negotiated ahead of
> time and used for many requests. For other uses, though it is nice to be able
> to pass a reference to a given schema using a small id and allow this to be
> looked up. Since only a small number of schemas are likely to be active for a
> given data source, these can easily be cached, so the number of remote
> lookups is very small (one per active schema version).
> Basically this would consist of two things:
> 1. A simple REST service that stores and retrieves schemas
> 2. Some helper java code for fetching and caching schemas for people using
> the registry
> We have used something like this at LinkedIn for a few years now, and it
> would be nice to standardize this facility to be able to build up common
> tooling around it. This proposal will be based on what we have, but we can
> change it as ideas come up.
> The facilities this provides are super simple, basically you can register a
> schema which gives back a unique id for it or you can query for a schema.
> There is almost no code, and nothing very complex. The contract is that
> before emitting/storing a record you must first publish its schema to the
> registry or know that it has already been published (by checking your cache
> of published schemas). When reading you check your cache and if you don't
> find the id/schema pair there you query the registry to look it up. I will
> explain some of the nuances in more detail below.
> An added benefit of such a repository is that it makes a few other things
> possible:
> 1. A graphical browser of the various data types that are currently used and
> all their previous forms.
> 2. Automatic enforcement of compatibility rules. Data is always compatible in
> the sense that the reader will always deserialize it (since they are using
> the same schema as the writer) but this does not mean it is compatible with
> the expectations of the reader. For example if an int field is changed to a
> string that will almost certainly break anyone relying on that field. This
> definition of compatibility can differ for different use cases and should
> likely be pluggable.
> Here is a description of one of our uses of this facility at LinkedIn. We use
> this to retain a schema with "log" data end-to-end from the producing app to
> various real-time consumers as well as a set of resulting AvroFile in Hadoop.
> This schema metadata can then be used to auto-create hive tables (or add new
> fields to existing tables), or inferring pig fields, all without manual
> intervention. One important definition of compatibility that is nice to
> enforce is compatibility with historical data for a given "table". Log data
> is usually loaded in an append-only manner, so if someone changes an int
> field in a particular data set to be a string, tools like pig or hive that
> expect static columns will be unusable. Even using plain-vanilla map/reduce
> processing data where columns and types change willy nilly is painful.
> However the person emitting this kind of data may not know all the details of
> compatible schema evolution. We use the schema repository to validate that
> any change made to a schema don't violate the compatibility model, and reject
> the update if it does. We do this check both at run time, and also as part of
> the ant task that generates specific record code (as an early warning).
> Some details to consider:
> Deployment
> This can just be programmed against the servlet API and deploy as a standard
> war. You have lots of instances and load balance traffic over them.
> Persistence
> The storage needs are not very heavy. The clients are expected to cache the
> id=>schema mapping, and the server can cache as well. Even after several
> years of heavy use we have <50k schemas, each of which is pretty small. I
> think this part can be made pluggable and we can provide a jdbc- and
> file-based implementation as these don't require outlandish dependencies.
> People can easily plug in their favorite key-value store thingy if they like
> by implementing the right plugin interface. Actual reads will virtually
> always be cached in memory so this is not too important.
> Group
> In order to get the "latest" schema or handle compatibility enforcement on
> changes there has to be some way to group a set of schemas together and
> reason about the ordering of changes over these. I am going to call the
> grouping the "group". In our usage it is always the table or topic to which
> the schema is associated. For most of our usage the group name also happens
> to be the Record name as all of our schemas are records and our default is to
> have these match. There are use cases, though, where a single schema is used
> for multiple topics, each which is modeled independently. The proposal is not
> to enforce a particular convention but just to expose the group designator in
> the API. It would be possible to make the concept of group optional, but I
> can't come up with an example where that would be useful.
> Compatibility
> There are really different requirements for different use cases on what is
> considered an allowable change. Likewise it is useful to be able to extend
> this to have other kinds of checks (for example, in retrospect, I really wish
> we had required doc fields to be present so we could require documentation of
> fields as well as naming conventions). There can be some kind of general
> pluggable interface for this like
> SchemaChangeValidator.isValidChange(currentLatest, proposedNew)
> A reasonable implementation can be provided that does checks based on the
> rules in http://avro.apache.org/docs/current/spec.html#Schema+Resolution. Be
> default no checks need to be done. Ideally you should be able to have more
> than one policy (say one treatment for database schemas, one for logging
> event schemas, and one which does no checks at all). I can't imagine a need
> for more than a handful of these which would be statically configured
> (db_policy=com.mycompany.DBSchemaChangePolicy,
> noop=org.apache.avro.NoOpPolicy,...). Each group can configure the policy it
> wants to be used going forward with the default being none.
> Security and Authentication
> There isn't any of this. The assumption is that this service is not publicly
> available and those accessing it are honest (though perhaps accident prone).
> These are just schemas, after all.
> Ids
> There are a couple of questions about ids how we make ids to represent the
> schemas:
> 1. Are they sequential (1,2,3..) or hash based? If hash based, what is
> sufficient collision probability?
> 2. Are they global or per-group? That is, if I know the id do I also need to
> know the group to look up the schema?
> 3. What kind of change triggers a new id? E.g. if I update a doc field does
> that give a new id? If not then that doc field will not be stored.
> For the id generation there are various options:
> - A sequential integer
> - AVRO-1006 creates a schema-specific 64-bit hash.
> - Our current implementation at LinkedIn uses the MD5 of the schema as the id.
> Our current implementation at LinkedIn uses the MD5 of the schema text after
> removing whitespace. The additional attributes like doc fields (and a few we
> made up) are actually important to us and we want them maintained (we add
> metadata fields of our own). This does mean we have some updates that
> generate a new schema id but don't cause a very meaningful semantic change to
> the schema (say because someone tweaked their doc string), but this doesn't
> hurt anything and it is nice to have the exact schema text represented. An
> example of uses these metadata fields is using the schema doc fields as the
> hive column doc fields.
> The id is actually just a unique identifier, and the id generation algorithm
> can be made pluggable if there is a real trade-off. In retrospect I don't
> think using the md5 is good because it is 16 bytes, which for a small message
> is bulkier than needed. Since the id is retained with each message, size is a
> concern.
> The AVRO-1006 fingerprint is super cool, but I have a couple concerns
> (possibly just due to misunderstanding):
> 1. Seems to produce a 64-bit id. For a large number of schemas, 64 bits makes
> collisions unlikely but not unthinkable. Whether or not this matters depends
> on whether schemas are versioned per group or globally. If they are per group
> it may be okay, since most groups should only have a few hundred schema
> versions at most. If they are global I think it will be a problem.
> Probabilities for collision are given here under the assumption of perfect
> uniformity of the hash (it may be worse, but can't be better)
> http://en.wikipedia.org/wiki/Birthday_attack. If we did have a collision we
> would be dead in the water, since our data would be unreadable. If this
> becomes a standard mechanism for storing schemas people will run into this
> problem.
> 2. Even 64-bits is a bit bulky. Since this id needs to be stored with every
> row size is a concern, though a minor one.
> 3. The notion of equivalence seems to throw away many things in the schema
> (doc, attributes, etc). This is unfortunate. One nice thing about avro is you
> can add your own made-up attributes to the schema since it is just JSON. This
> acts as a kind of poor-mans metadata repository. It would be nice to have
> these maintained rather than discarded.
> It is possible that I am misunderstanding the fingerprint scheme, though, so
> please correct me.
> My personal preference would be to use a sequential id per group. The main
> reason I like this is because the id doubles as the version number, i.e.
> my_schema/4 is the 4th version of the my_schema record/group. Persisted data
> then only needs to store the varint encoding of the version number, which is
> generally going to be 1 byte for a few hundred schema updates. The string
> my_schema/4 acts as a global id for this. This does allow per-group sharding
> for id generation, but sharding seems unlikely to be needed here. A 50GB
> database would store 52 million schemas. 52 million schemas "should be enough
> for anyone". :-)
> Probably the easiest thing would be to just make the id generation scheme
> pluggable. That would kind of satisfy everyone, and, as a side-benefit give
> us at linkedin a gradual migration path off our md5-based ids. In this case
> ids would basically be opaque url-safe strings from the point of view of the
> repository and users could munge this id and encode it as they like.
> APIs
> Here are the proposed APIs. This tacitly assumes ids are per-group, but the
> change if pretty minor if not:
> Get a schema by id
> GET /schemas/<group>/<id>
> If the schema exists the response code will be 200 and the response body will
> be the schema text.
> If it doesn't exist the response will be 404.
> GET /schemas
> Produces a list of group names, one per line.
> GET /schemas/group
> Produces a list of versions for the given group, one per line.
> GET /schemas/group/latest
> If the group exists the response code will be 200 and the response body will
> be the schema text of the last registered schema.
> If the group doesn't exist the response code will be 404.
> Register a schema
> POST /schemas/groups/<group_name>
> Parameters:
> schema=<text of schema>
> compatibility_model=XYZ
> force_override=(true|false)
> There are a few cases:
> If the group exists and the change is incompatible with the current latest,
> the server response code will be 403 (forbidden) UNLESS the force_override
> flag is set in which case not check will be made.
> If the server doesn't have an implementation corresponding to the given
> compatibility model key it will give a response code 400
> If the group does not exist it will be created with the given schema (and
> compatibility model)
> If the group exists and this schema has already been registered the server
> returns response code 200 and the id already assigned to that schema
> If the group exists, but this schema hasn't been registered, and the
> compatibility checks pass, then the response code will be 200 and it will
> store the schema and return the id of the schema
> The force_override flag allows registering an incompatible schema. We have
> found that sometimes you know "for sure" that your change is okay and just
> want to damn the torpedoes and charge ahead. This would be intended for
> manual rather than programmatic usage.
> Intended Usage
> Let's assume we are implementing a put and get API as a database would have
> using this registry, there is no substantial difference for a messaging style
> api. Here are the details of how this works:
> Say you have two methods
> void put(table, key, record)
> Record get(table, key)
> Put is expected to do the following under the covers:
> 1. Check the record's schema against a local cache of schema=>id to get the
> schema id
> 3. If it is not found then register it with the schema registry and get back
> a schema id and add this pair to the cache
> 4. Store the serialized record bytes and schema id
> Get is expected to do the following:
> 1. Retrieve the serialized record bytes and schema id from the store
> 2. Check a local cache to see if this schema is known for this schema id
> 3. If not, fetch the schema by id from the schema registry
> 4. Deserialize the record using the schema and return it
> Code Layout
> Where to put this code? Contrib package? Elsewhere? Someone should tell me...
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