HTTP API – Project Haystack


OverviewAuthenticationURI NamespaceRequestsResponsesError HandlingError GridIncomplete DataContent NegotiationWatches


The Haystack HTTP API defines a simple mechanism to exchange haystack tagged data over HTTP.

A haystack HTTP API server implements a set of ops or operations. An operation is a URI that receives a request and returns a response. Standard operations are defined to query the entity database, setup subscriptions, or read/write history time-series data. Operations are pluggable so that vendors can enhance their API interface with their own customized, value-added functionality.

Both requests and responses are modeled as grids. Grids are encoded using one the standard MIME types for grid serialization, or may be pluggable using HTTP content negotiation.


Compliant HTTP API implementations must implement the authentication protocol specified in the Authentication chapter.

URI Namespace

A haystack server defines a HTTP URI as its base address. Operations are then mapped as path names under that address. Example:

http://server/haystack/           // base URI
http://server/haystack/{op}       // operation URI pattern
http://server/haystack/about      // about op
http://server/haystack/read       // read op

The base URI must be assumed to end with a trailing slash even if it is not always expressed that way.


A client makes a request to a server by sending a single grid and the server responds with a grid. Typically grids are encoded using Zinc or Json, but the actual encoding used is pluggable using content negotiation.

GET Requests

If an op is tagged with the noSideEffects marker, then it may be called with a HTTP GET request as long as the operation requires no arguments or a single row request. The request grid is encoded in the HTTP query string where each query parameter is mapped to tags in a single row. The tag value must be Zinc encoded, otherwise it is assumed to be of a Str type. Using a GET request on an op without the noSideEffects tag must return a 405 Method Not Allowed error.

Example of request with empty grid:

// GET request URI

// POST request grid in Zinc

// POST request grid in JSON
"_kind": "grid",
"meta": {"ver":"3.0"},
"cols": [{"name":"empty"}],

Example of request with single Str tag:

// GET request URI

// POST request grid in Zinc

// POST reqeust grid in JSON
"_kind": "grid",
"meta": {"ver":"3.0"},
"cols": [{"name":"filter"}],

Example of request with multiple tags encoded as Zinc:

// GET request URI

// POST request grid in Zinc

// POST request grid in JSON
"_kind": "grid",
"meta": {"ver":"3.0"},
"cols": [{"name":"id"},{"name":"range"}],
"rows":[{"id":{"_kind":"ref", "val":"hisId"}, "range":"yesterday"}]

POST Requests

If an op is not tagged noSideEffects or the request grid is anything other than a single row of name/value pairs, then it must be be sent using HTTP POST. The client must encode the grid using a MIME type supported by server. The client can query the supported MIME types using the filetypes op. The following is an example of posting to the hisRead op using Zinc:

POST /haystack/hisRead HTTP/1.1
Content-Type: text/zinc; charset=utf-8
Content-Length: 39



If the request grid is successfully read by the server, then it processes the operation and returns the HTTP status code 200 and serializes the response result as a MIME encoded grid.

Error Handling

There are three type of errors that may occur when a client makes a request to a server:

  • Network I/O errors
  • HTTP errors
  • Request errors

Network errors occur when the client cannot make a successful TCP connection to the server. This might include invalid host name, port number, or network outage. Typically, these sorts of errors are raised as I/O exceptions by the client's runtime.

HTTP errors occur when TCP connections can be successfully established, but the server cannot successfully handle the URI or request grid at the HTTP layer. The following HTTP status codes must be used in these cases:

  • 400: the client failed to specify a required header such "Content-Type"
  • 403: the client is not authorized to access the op
  • 404: the URI does not map to a valid operation URI
  • 405: attempt to use GET with an op not tagged noSideEffects
  • 406: the client "Accept" header requested an unsupported MIME type
  • 415: the client posted the request grid using an unsupported MIME type
  • 501: the HTTP method is other than "GET" or "POST"

Request errors occur after the server has successfully read the request grid. If the server cannot fulfill the request for any reason, then it must return HTTP response code of 200 with a error grid as the body. HTTP error codes must only be used if the request grid itself cannot be read. Request errors include but are not limited to:

  • invalid request grid data columns
  • invalid request grid data types
  • unknown or invalid entity identifiers
  • inconsistent data types or data values
  • server internal errors/exceptions

Error Grid

If an operation failed after a request grid is successfully read by a server, then the server returns an error grid. An error grid is indicated by the presence of the err marker tag in the grid metadata. All error grids must also include a dis tag in the grid metadata with a human readable descripton of the problem. If the server runtime supports stack traces, this should be reported as a multi-line string via the errTrace tag in the grid metadata.

Example of an error grid:

ver:"3.0" err dis:"Cannot resolve id: badId" errTrace:"UnknownRecErr: badId\n  ...."

Clients must always check for the present of the err grid marker tag to determine if the reponse is an error or a valid result.

Incomplete Data

Sometimes a HTTP request will be successfully processed, but will return incomplete data. This will often happen because of throttling based on timeouts or limits on the amount of data returned by one request. When this occurs, the response should add an incomplete tag to the response grid meta. The value of incomplete must be a Dict and should use the following tags:

  • dis: Str message to display to the user about why data is incomplete 
  • limit: Number if incomplete because of threshold limit 
  • timeout: Number duration if incomplete because of timeout

Example of incomplete grid:

ver:"3.0" incomplete:{dis:"Request timeout exceeded!" timeout:1min}
colA, colB, colC
"a", "b", "c"

Content Negotiation

The default for all HTTP API operations is to return the result as a grid (or grids) with the MIME type "text/zinc" formatted using Zinc. You can request to receive the results in alternate formats by specifying the "Accept" header in your HTTP request.

The following "Accept" header MIME types are standardized:

  • Zinc: text/zinc, */*, or unspecified
  • Json: application/json, or application/vnd.haystack+json;version=4
  • JSON v3: application/vnd.haystack+json;version=3
  • Trio: text/trio
  • CSV: text/csv
  • Turtle: text/turtle
  • JSON-LD: application/ld+json

If you specify an "Accept" header for an unsupported MIME type, then the 406 Unacceptable error code is returned.

Example for reading all site entities as CSV:

GET /haystack/read?filter=site
Accept: text/csv


HTTP/1.1 200 OK
Content-Type: text/csv; charset=utf-8

Site A,2000ft²,"1000 Main St,Richmond,VA"
Site B,3000ft²,"2000 Cary St,Richmond,VA"

All text media type must be be encoded using UTF-8.


Haystack adopts the watch design from oBIX to handle real-time subscriptions. Watches are a stateful polling mechanism designed to work efficiently over HTTP.

The life cycle of a watch is as follows

  1. The client creates a new watch using the watchSub operation
  2. The client polls for changed entities using the watchPoll operation
  3. The client explicitly closes the watch using watchUnsub or the server may automatically close the watch if the client fails to poll after a lease period

During the lifetime of a watch, the client may optionally:

  1. add new entities to the watch using watchSub
  2. remove entities from the watch using watchUnsub
  3. perform a poll refresh to re-read all the watched entities using watchPoll