ElasticSearch Guided (code) Generator

Development

Install dependencies with opam install --deps-only .

Build with make

Mapping

esgg takes as an input an ES mapping (schema description) and actual query (with syntax for variables, described below). Often additional information is needed to map ES fields into proper OCaml types, this is achieved by attaching _meta annotation object to the affected field (ES only supports _meta at root level, so these annotations make it impossible to store extended mapping back into ES which is a pity), as follows:

    "counts": {
      "_meta": {
        "optional": true
      },
      "properties": {
        "hash": {
          "type": "long",
          "_meta": { "repr": "int64" }
        },
        "value": {
          "type": "long"
        }
      }
    },

Supported _meta attributes:

• {"list":true} - property is an array (mapped to list)
• {"list":"sometimes"} - property is either an array or single element (mapped to json with custom ocaml module wrap that will need to be provided in scope)
• {"optional":<true|false>} - property may be missing (mapped to option)
• {"ignore":true} - skip property altogether
• {"fields_default_optional":true} - any subfield may be missing (can be overriden by per-field optional:false)
• {"repr":"int64"} - override ES type, currently the only possible value is "int64" to ensure no bits are lost (by default long is mapped to OCaml int)

Host types mapping

Generated code allows to use application types for any fields. This is achieved by referencing specific type for each field in generated code, instead of the primitive type from the mapping, allowing consumer of the code to map it onto custom type etc. For example the field hash in example above will have type Counts.Hash.t in generated code. In order to compile the generated code this type must be present in scope and mapped to something useful. Default mapping (which just maps everything to corresponding primitive types) can be generated with esgg reflect <mapping name> <mapping.json>, e.g.:

esgg reflect hello_world src/mappings/hello_world.json >> src/mapping.ml

will generate the following, which should be edited manually as needed, e.g. by making Hash a module with an abstract type

  module Counts = struct
    module Hash = Id_(Int64_)
    module Value = Id_(Long_)
  end

Variables

Syntax for variables in template json files is as follows:

• $var for regular required variable
• $var? for optional variable (minimal surrounding scope is conditionally expunged)
• full form $(var:hint) where hint can be either list or list? currently

Reusing shared ATD definitions

To reuse shared definitions using the -shared <file.atd> option, the atd file must have the <esgg from="..."> annotation at the top of the file. The value of the annotation must correspond to the OCaml module containing the shared definitions.

Example:

# file.atd

<esgg from="Your_ocaml_module_name">

...atd type definitions...

Elasticsearch features

TODO document what is supported

Some notes follow:

aggregations

The following aggregation types are supported:

filters

• named
• anonymous
• dynamic (i.e. a variable)
• partial dynamic (i.e. containing variables)
• other_bucket and other_bucket_key
• other_bucket with anonymous filters (ignored, user is responsible to treat last element of result specially)

Dynamic (defined at runtime) filters are supported, as follows { "filters": { "filters": $x } }. In this case corresponding part of output will be quite untyped. $x is assumed to be a dictionary and result will be represented with dictionaries. For anonymous filters (ie array of filters) use $(x:list).

date_histogram

key_as_string is returned in output only when format is explicitly specified, to discourage fragile code.

range

Keyed aggregation expects explicit key for each range. from/to fields in response are not extracted.

date_range

same as for range aggregation

others

• min
• max
• avg
• sum
• value_count
• cardinality
• terms
• significant_terms
• significant_text
• histogram
• top_hits
• nested
• reverse_nested
• bucket_sort

dynamic

Specifying aggregation as variable ($var) will lead to an untyped json in place of aggregation output, this can be used as temporary workaround for unsupported aggregation types or for truly dynamic usecase (aggregation built at run-time).

script

Scripts are opaque, ie no type information is extracted and result is json.

source filtering

• exclude and include
• wildcards
• dynamic (i.e. a variable) NB not implemented for get and mget

Tests

make test runs regression tests in test/ verifying that input and output atd generated from query stays unchanged. Once there is an expected change in generated query - it should be committed. Tests are easy to add and fast to run.

TODO tests to verify that:

• code generated for query application of input variables does actually compile and produce correct query when run
• atd description of output (generated from query) can indeed unserialize ES output from that actual query

Conditions

Copyright (c) 2018 Ahrefs github@ahrefs.com

This project is distributed under the terms of GPL Version 2. See LICENSE file for full license text.

NB the output of esgg, i.e. the generated code, is all yours of course :)

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2019-10-29