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Programming language: Haskell
License: BSD 3-clause "New" or "Revised" License
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This is a snaplet for Haskell Snap framework which provides create-read-update-delete operations for any JSON objects using Redis storage. JSON objects are mapped to Redis hashes. Role-based permissions system is supported.

Primary notion of Redson is a model, or a form, which is a named collection of fields with certain extra annotations. Every filled form («instance of model») has unique ID when stored in Redis.

Models must be defined prior to usage with special syntax, which may also be requested by client code to build interface for models (using JavaScript templating, for example).

Transparent mode is supported in which model definitions are not checked, but permissions engine is unavailable as well. Certain transformations may be applied to model definition by server and served to client (see «served models»).


Prior to reading and writing form data, the server reads form (model) definition from JSON files stored in directory set by models-directory setting (defaults to resources/models/), unless transparent-mode setting is set to true (see below).

Form definition example

(As described in The SCP Foundation archives)

    "name": "SCP",
    "canCreate": ["front"],
    "canRead": true,
    "canUpdate": ["front", "back", "manager"],
    "canDelete": ["manager"],
    "fields": [
            "name": "code",
            "meta": {
                "label": "Code"
            "canWrite": ["front", "back"]
            "name": "title",
            "meta": {
                "label": "Title"
            "canWrite": ["front", "back"]
            "name": "foundAt",
            "meta": {
                "label": "Original location"
            "canWrite": ["front", "back"]
            "name": "dangerClass",
            "meta": {
                "label": "Danger class",
                "dictionaryName": "DangerClasses",
                "default": "lev0"
            "type": "dictionary",
            "canRead": ["front"],
            "canWrite": ["back"]
            "name": "conditions",
            "meta": {
                "label": "Special Containment Procedures"
            "type": "textarea",
            "canWrite": ["back"]
            "name": "description",
            "type": "textarea",
            "meta": {
                "label": "Description"
            "canWrite": ["back"]

Valid keys for form and fields are listed below. Every field is required unless explicitly stated otherwise.

Model name

Consider a model is stored in scp.js file; we define model name as the filename without extension (scp). Model name is used to construct the URL to access forms built from this model.

Valid form definition keys

  1. name

    Internal name of model. Must match the basename of JSON file model definition is stored in.

  2. title

    Human-readable form title.

  3. canCreate / canUpdate / canRead / canDelete (optional)

    Each field lists roles having access to specific CRUD methods for model. Boolean values may be used instead of role lists as well, with false meaning «none of the roles» and true meaning «all the roles». Empty list is the same as false.

    Implementation follows these rules:

- `canCreate` implies `canUpdate` permission.

Note that per-field permissions are still checked after whole-form
ones succeed.

If field is not included in model definition, `false` is implied.

Attempt to violate permissions results in 403 Forbidden error.
  1. fields

    List of field objects, where every field object has keys listed below.

  2. applications

    List of application objects which target many fields at once (see below).

Valid field definition keys

  1. name

    Field object must include name key which is the internal name of the field. Name is alphanumeric string.

    class cannot be used as a field name.

  2. type (optional)

    Type system currently does not impose any checks on server side. Following values are proposed for type:

-   `text`,
-   `textarea`,
-   `checkbox`,
-   `dictionary`,
-   `reference`

Default field type is `text`

Client uses type data to properly render field contents.

If type of field is `reference`, then field holds a number of
references to another form instances. The client may provide
interface to edit or view the subform. The actual value provided
for the form field with `reference` type should be comma-separated
list of `<formname>:<instanceid>` strings, where `<formname>` is
the name of form to which the reference is stored and
`<instanceid>` is the id of specific instance of that form.
  1. canRead / canWrite (optional)

    Per-field permissions have the same syntax as per-field per-form permissions. Only readable fields are served to client and only writable are allowed to be changed by client.

    Implementation generally follows these rules:

-   If field is unreadable, then client must not render it.

-   If field is not writable by user, then form input element must
    be inactive.

-   `canWrite` implies `canRead`.

-   Client does not attempt to write inaccessible fields to server
    and does not expect unreadable fields in server response.
  1. groupName (optional)

    If groupName key is present, its values must be equal to name of one of the complex field groups as described in group fields file. When server processes model description, such fields are spliced into list of fields as described for that group (see below).

  2. index (optional)

    Boolean which instructs server to create or update exact reverse index on that field on create/update/delete operations.

    Redis key used is <modelname>:<field>:<value>, value stored is a set of instance ids with that value for indexed field.

  3. indexCollate (optional)

    If field has index=true, then indexCollate instructs Redson whether or not to perform string cleaning (removing punctuation, space, lower-case conversion) prior to saving index values and performing an ad-hoc search against them. Defaults to false. Set this to true if you want to search by names or other user-provided strings.

    This flag may be removed in the upcoming releases.

  4. meta (optional)

    A hash of arbitary keys and values which are not treated by server in any way. These may be used by a client for field-specific behaviour or templating.

    Suggested meta annotations:

    1. default

      Specifies the default value of field. (Will possibly move out of meta if server will enforce default values upon creation some day).

    2. dictionaryName

      When field type is dictionary, dictionaryName key must be present in field description as well. Client should provide certain mapping between stored field contents and displayed value using the name of dictionary.

    3. invisible

      Boolean field to indicate that field should not be rendered on UI (or not handled by client at all).

    4. label

      Human-readable field label.

    5. readonly

      Boolean field to indicate that client must render field as disabled.

    6. required

      Boolean field to indicate that field is required. Client may provide custom checks in UI for such fields. (Will possibly move out of meta)

Group fields

A group of fields (complex field) with distinctive name may be shared across several models. Valid complex fields must be defined in a file set by field-groups-file setting ("resources/field-groups.json"), which must contain a JSON hash where keys are group names and values are fields in respective group.

    "address": [
            "name": "city",
            "meta": {
                "label": "City"
            "name": "zip",
            "meta": {
                "label": "ZIP / Postal code"
            "name": "address",
            "meta": {
                "label": "Address"
            "type": "textarea"

Using groupName in field description is not allowed (no recursive complex fields).


Per-field permissions (set in canRead and canWrite field properties) are checked prior to writing any data to Redis or sending response to client (unless transparent-mode is true). Implementation currently follows these rules:

  • No unreadable fields are sent to client on READ methods;
  • Attempt to perform any operation without being logged in results in 401 Unauthorized error.
  • Attempt to perform any operation on unknown model results in 404 Not Found error;
  • Attempt to create or update instances with unwritable fields will be rejected with 403 Forbidden.

Field applications

It's possible to change certain annotations for many fields in model with one instruction. application key of form definition contains a list of application objects. Every application object may contain canRead, canWrite and meta keys with same syntax as in fields. Additionally, targets keys must be present. If targets is a list of field names, then new values for canRead, canWrite are set for matched fields. true value of targets matches every field. meta values from application and matched field are merged, with meta keys from application having precedence over field meta.

Example (set new label and foo meta, new permissions for all fields):

    "targets": true,
    "meta": {
        "label": "Renamed label",
        "foo": "bar"
    "canRead": ["changed_role"],
    "canWrite": false

Example (change label of "foo" field):

    "targets": ["foo"],
    "meta": {
        "label": "Foo field"

Served models

Client may request stripped form description by sending this request:

GET /<modelname>/model/

Server takes several steps to serve the model.

  1. Permissions processing

    Response will contain original description but without fields unreadable by current user. canWrite field property will be set to boolean value for every form field, indicating whether the current user can write to this field. Whole-form permissions will be set to booleans as well, indicating whether the current user has specific permissions.

  2. Meta bags

    meta for every field as served as-is without any changes.

  3. Group splicing

    Every field f with groupName annotation is spliced into list of actual group members in served model, and groupName property is attached to every field in splice result, with value equal to name of group. Name of every field in group is prepended with f_, where f is the name of original field which was spliced into group. Client may use this data to recognize fields from the same group and render them specifically.


    Assuming group bar has fields named f1, f2 and f3, and model has field with group splice annotation:

    "name": "foo",
    "groupName": "bar"

    then foo will be spliced into fields named foo~f1~, foo~f2~ and foo~f3~, and groupName for all these fields will be set to bar.

  4. Applications

    Applications are performed (in sequence following the order they're listed in model definition) after group splicing, which means applications may be used to override default field annotations set for group members in field-groups-file.

  5. Index fields list caching

    Served form will also contain indices field which is a list of index fields of model.

  6. Readable models

    Client may also request list of readable models from

    GET /_models/


We implement generic CRUD for our forms using Redson snaplet, which is best explained by its routes (assuming they're installed in top-level snaplet under /_ URL; prefix may be changed using nestSnaplet in parent initializer).

All interactions with server use JSON objects as primary format. JSON objects are mapped to Redis hashes stored under <modelname>:<id> key. Fresh ID's are provided by using global:<modelname>:<id> key which is INCR-ed after every new form instance is created in Redis.

No schema checking is performed by server, but permissions engine will disallow writing arbitary models and fields to server.

CRUD mapping to HTTP methods is implemented in Redson as expected by Backbone:

CREATE → POST /<modelname>
READ → GET /<modelname>/<instanceid>/
UPDATE → PUT /<modelname>/<instanceid>/
DELETE → DELETE /<modelname>/<instanceid>/

Redson snaplet is parametrized by AuthManager snaplet lens (usually from top-level application). All methods implemented by Redson require the user to be logged in, 401 Unauthorized HTTP error response is issued otherwise.

Server interface by example

Assume we're using scp.js model given above.


    Server request:

    curl localhost:8000/_/scp/ -X POST -d "{\"title\":\"Able\", \"code\":\"076\", \"class\":\"Keter\"}"

    What server did in Redis:

    incr global:scp:id

    (24 is returned)

    hmset scp:24 code 076 title Able class Keter

    Server response:


    (note the id field which is returned by server after Redis was updated with new form instance. Backbone stores new instance id upon receiving server response and uses it in further server requests for saving updated model instance)

  2. READ

    Server request:

    curl localhost:8000/_/scp/24/ -X GET

    Server response:


    Redis command used:

    hgetall scp:24

    What is sent to server:

    curl localhost:8000/_/scp/24/ -X PUT -d "{\"title\":\"Able\", \"code\":\"076-2\", \"class\":\"Keter\", \"description\":\"Really nasty thing\"}"

    Server response is 204 (success, No content) in case the instance previously existed and 404 if not.

    Note that the all model fields are sent to server (this may be improved for efficiency).


    Server request:

    curl localhost:8000/_/scp/24/ -X DELETE

    Redis deletes the key:

    del scp:24

    Server response contains JSON of instance before deletion:

    {"code":"076-2","title":"Able","description":"Really nasty thing","class":"Keter"}


Search interface for model <modelname> is available under /_/<modelname>/search access point via GET method. canRead form permission is required to search for instances.

Accepted parameters are:

  • key-value pairs where keys are index fields of model and values are search terms;

  • limit parameter which sets maximum number of items served;

  • matchType=p or matchType=s for prefix search or substring search of value in index field (prefix search is faster);

  • searchType=and or searchType=or which indicates if all search terms must match or just any of them.

  • fields=f1,f2,f3 which is a list of fields which must be extracted from every matched instance and served in response.

Response is a list of JSON objects for matched instances. If fields is provided, then response is a list of arrays instead, where every array contains values of specified fields in instance (in order given by fields parameter; if value is not present then null is used).

No per-field read permissions are checked.

Currently search is implemented using slow keys Redis command, and should be considered an ad-hoc solution only.

Extra features


There's an extra entity stored in Redis for every model called timeline, which is a list with id's of instances stored in DB (in order of creation).

/_/scp/timeline/ serves JSON list of last N (currently 10) timeline items for model "scp":

curl localhost:8000/_/scp/timeline/ -X GET

If instance is removed from Redis, corresponding timeline entry is removed as well.

Client front-end uses timeline to show links to fresh instances.

canRead model permission is required to access model timeline.

WebSockets notifications

/_/<modelname>/events/ provides instance creation/deletion notifications through WebSockets interface. Events are transmitted to clients in JSON format with fields event, model and id, where event is either create or delete. No permissions are checked currently when accessing events.

Snapless operation

The package provides Snap.Snaplet.Redson.Snapless.* modules:

  • CRUD — low-level operation with Redis DB (on commit level, where commit is a list of key-value pairs for named hash); operations support index updates so use this for tools which need to fiddle with Redson indices;

  • Metamodel — model definition parsing;

  • Loader — load models from filesystem locations, splicing groups; served models are provided using this module.

Redis interface

We use redis bindings provided by snaplet-redis package. Pool size numbers are yet to be tuned.


Following config options are recognized by Redson:

  • models-directory ("resources/models"): directory which contains model definitions to be read by Redson.

  • field-groups-file ("resources/field-groups.json"): file which contains descriptions of usable complex field groups.

  • transparent-mode (false): when true, no permissions checking is performed. Redson acts in «transparent mode» allowing to store and retrieve any JSON data. Any model may be written to.

To do

Cache user permissions

Snap.Snaplet.Redson.Snapless.Metamodel coupled with withCheckSecurity provides permissions checking upon every CRUD operation. Intersecting user roles and role lists set in form/field permissions should be performed once when first request from that user is received and cached for all further requests (models can't be changed without Redson restart anyways, and restart will be required when new users are added as well).

External search providers

Might subscribe to model events via socket. Provides lists of matching instance ids.

Update inverse references

When instance of model becomes referenced by another instance, inverse reference should be updated by server.

We already have indices out of the box so we can get this feature for free.

Perhaps orphan dependent models should be cleared if parent is deleted.

Factor out Snap.Snaplet.Redson.Util to snap-errors module

Force default values when creating instance

Configurable pool size

WebSockets interface improvement

  • [X] load-model.js contains full URI to WebSockets entry point (currently hardcoded for scp model)
  • [ ] publish events only for respective model under <model>/events entry point (requires addressing extension for PubSub or multiple PubSubs; non-transparent mode only + BigBrother role for «all models» event entry point)
  • [ ] check permissions (if we want to serve associated commits in event messages, per-model canRead may be not enough (what if listener has no access to certain fields and we can't strip commit for every listener personally))
  • [ ] possibly use native Redis' publish/subscribe mechanism

Support search in transparent mode

Currently only index fields of model are searched against, which means that model definition must be available for searching (this also required redundant name field in form definition), while in fact redisSearch can be implemented using ModelName only. External providers must support name-only operation as well.