Rather than using a fully custom way of representing the resources we expose in
Horizon, we use HAL. HAL is a
hypermedia format in JSON that remains simple while giving us a couple of
benefits such as simpler client integration for several languages. See this
wiki page for a
list of libraries.
Attributes, Links, Embedded Resources
At its simplest, a HAL response is just a JSON object with a couple of reserved
_links is used for expressing links and
_embedded is used
for bundling other HAL objects with the response. Other than links and embedded
objects, HAL is just JSON.
HAL is a hypermedia format, like HTML, in that it has a mechanism to express
links between documents. Let’s look at a simple example:
The above response is for the genesis ledger of the Stellar test network, and
the links in the
_links attribute provide links to other relavant resources in
Horizon. Notice the object beneath the
transactions key. The key of each
link specifies that links relation to the current resource, and in this case
transactions means “Transactions that occurred in this ledger”. Logically,
you should expect that resource to respond with a collection of transactions
with all of the results having a
ledger_sequence attribute equal to 1.
transactions link is also templated, which means that the
attribute of the link is actually a URI template, as specified by RFC
6570. We use URI templates to show you
what parameters a give resource can take. You must evaluate the template to a
valid URI before navigating to it.
Pages represent a subset of a larger collection of objects.
As an example, it would be unfeasible to provide the
All Transactions endpoint without paging. Over time there
will be millions of transactions in the Stellar network’s ledger and returning
them all over a single request would be unfeasible.
Read more about paging in following docs:
Certain endpoints in Horizon can be called in streaming mode using Server-Sent Events. This mode will keep the connection to horizon open and horizon will continue to return responses as ledgers close. All parameters for the endpoints that allow this mode are the same. The way a caller initiates this mode is by setting
Accept: text/event-stream in the HTTP header when you make the request.
You can read an example of using the streaming mode in the Follow Received Payments tutorial.
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