Hayata

Hayata is an Erlang/OTP framework for building AI agents around a graph-based execution model. It uses OTP processes, supervision, monitoring, and message passing to coordinate agent execution, with explicit ownership of process state and durable recovery checkpoints.

Hayata is designed to be usable from Erlang and Elixir applications. Its graph-oriented model is inspired by agent orchestration frameworks such as LangGraph, while exposing an Erlang/OTP-native runtime rather than a feature-for-feature clone of another API.

Highlights

Graph-based agent execution with a configurable decision policy.
Sequential and parallel node execution.
OTP supervision and process monitoring for agents, nodes, edges, reducers, and edge watchers.
Asynchronous invocation APIs and result notifications.
Invocation checkpoints and task-result persistence in Mnesia.
Recovery of interrupted invocations, including parallel transitions.
Conservative handling of uncertain tasks: a task marked as running without a durable result is not silently re-executed.
Telemetry lifecycle events for agents, nodes, edges, and reducers.

Architecture

Agent --- coordinates invocations, applies the decision policy, tracks invocation state, and initiates recovery.
Node --- a supervised gen_server that owns local execution state, coordinates outgoing edges, tracks executions, and monitors edges and reducers.
Edge --- an anonymous gen_server that runs an edge task and reports its terminal result.
Edge watcher --- monitors the node and its edge. If the node terminates unexpectedly, the watcher terminates the edge so it does not remain orphaned.
Reducer --- a temporary process that collects edge results for an execution and produces a consolidated outcome.
Task store --- persists invocation checkpoints and task outcomes in Mnesia.
Node supervisor --- supervises node processes and restarts them after abnormal termination.

Requirements

Erlang/OTP 29.1.1 (target runtime)
Rebar3

Build and test

rebar3 compile
rebar3 eunit

Quick start

The following example mirrors a basic graph-agent pattern: define a node, define control flow, start the agent's supervision tree, and invoke it. This example uses a mock node; it does not call an LLM.

%% hello_hayata.erl
-module(hello_hayata).

-export([run/0]).

run() ->
    AgentId = hello_agent,

    MockNode = fun(State) ->
        Message = maps:get(input, State, <<"Hello">>),
        #{
            messages => [
                #{role => ai, content => <<"Hello, world!">>},
                #{role => user, content => Message}
            ]
        }
    end,

    Nodes = #{mock_node => MockNode},

    DecisionPolicy = fun(Context) ->
        case maps:get(step, Context) of
            0 ->
                {next, mock_node, <<"Hello">>};
            1 ->
                {finish, maps:get(state, Context)}
        end
    end,

    Config = #{
        id => AgentId,
        state => #{},
        nodes => Nodes,
        decision_policy => DecisionPolicy,
        max_steps => 3,
        execution_timeout => 5000
    },

    case hayata_sup:start_agent(Config) of
        {ok, AgentSupPid} ->
            try
                run_agent(AgentSupPid)
            after
                hayata_sup:stop_agent(AgentSupPid)
            end;
        {error, Reason} ->
            {error, {agent_start_failed, Reason}}
    end.

run_agent(AgentSupPid) ->
    case hayata_agent_sup:agent(AgentSupPid) of
        {ok, AgentPid} ->
            case hayata_agent:run(AgentPid, #{input => <<"Hello">>}, self()) of
                {ok, InvocationId} ->
                    await_result(InvocationId, 100);
                {error, Reason} ->
                    {error, Reason}
            end;
        {error, Reason} ->
            {error, Reason}
    end.

await_result(_InvocationId, 0) ->
    {error, notification_timeout};
await_result(InvocationId, Attempts) ->
    receive
        {hayata_agent_result, InvocationId, {ok, Result}} ->
            {ok, Result};
        {hayata_agent_result, InvocationId, {error, Reason}} ->
            {error, Reason}
    after 100 ->
        await_result(InvocationId, Attempts - 1)
    end.

Save the module as hello_hayata.erl in a location included in your Erlang code path, compile it, and call:

hello_hayata:run().

Invocation API

Hayata supports two invocation APIs:

hayata_agent:run(AgentPid, Input) starts an asynchronous invocation and returns {ok, InvocationId}. Retrieve the result later with hayata_agent:get_result/1.
hayata_agent:run(AgentPid, Input, SubscriptionPid) starts an asynchronous invocation and sends the terminal result to the subscription process.

The subscription receives one of these messages:

{hayata_agent_result, InvocationId, {ok, Result}}
{hayata_agent_result, InvocationId, {error, Reason}}

InvocationId correlates the message with its invocation. Success, execution errors, and timeouts are reported through this message format. If an invocation is rejected before starting---for example, because the agent is already running---run/3 returns an error without sending a terminal-result notification for that rejected request.

The subscription argument must be a PID; otherwise, run/3 returns {error, invalid_subscription_pid}.

The decision policy controls execution flow: {next, NodeId, Input} schedules a node, and {finish, Result} ends the invocation.

Parallel node execution

A decision policy can schedule multiple nodes in parallel by returning {next, NodeIds, Input}, where NodeIds is a non-empty list of distinct node identifiers.

All branch nodes receive the same input and current shared agent state. Their returned maps are collected independently and do not directly modify the shared state.

When every branch completes successfully, the policy receives the results through the parallel_results field in its context. Results are a list of maps in the same order as the requested node identifiers, regardless of completion order.

DecisionPolicy = fun(Context) ->
    case maps:get(step, Context) of
        0 ->
            {next, [customer, orders], undefined};
        2 ->
            [Customer, Orders] = maps:get(parallel_results, Context),
            {next, aggregate, #{
                customer => Customer,
                orders => Orders
            }};
        3 ->
            {finish, maps:get(state, Context)}
    end
end.

The parallel_results field is undefined before a parallel transition completes. After all branches succeed, it contains their ordered result maps and remains available to the policy that chooses the next transition. When the policy schedules another node or parallel transition, the previous results are cleared. A new invocation starts with parallel_results set to undefined.

If a branch fails or the invocation times out, the transition fails and pending branches are cancelled; incomplete parallel results are not published as a successful transition result.

Durable checkpoints and recovery

Hayata uses Mnesia to persist invocation checkpoints and task outcomes. The task store uses disk-backed Mnesia tables and synchronous transactions; completed task results and invocation checkpoints are flushed to the Mnesia log after writes.

A checkpoint captures the information required to identify and resume an invocation, including the invocation ID, current state and step, active execution, parallel results, and terminal result/error fields. When an agent starts and finds an unfinished invocation for its ID, it restores that invocation and reports recovery_required rather than accepting a new invocation on top of it.

Call hayata_agent:recover(AgentPid) to initiate recovery of the restored invocation.

Recovery classifies each task using its durable record:

Completed: reuse the persisted task result instead of executing that task again.
Not started: schedule the task for execution during recovery.
Uncertain: if a task record indicates that execution started but no completed result exists, recovery returns {error, task_execution_uncertain} rather than silently repeating the operation.

For parallel execution, Hayata can combine completed branch results with branches that have not started, execute only the missing branches, and persist the final invocation result. Results retain the configured node order.

Recovery guarantees and limitations

Recovery is designed to avoid silently repeating a task whose completion is uncertain. It does not provide exactly-once semantics for arbitrary external side effects. If a node may have called an external service before the VM stopped but its result was not durably saved, Hayata cannot know whether that side effect occurred. Applications should use idempotency keys, external deduplication, or other application-level safeguards where retry safety matters.

Durable recovery depends on preserving the configured Mnesia data directory across VM restarts. If that directory is removed or replaced, the persisted checkpoints are unavailable.

Reproducing VM restart scenarios

The repository includes VM_RECOVERY_TESTS.md, a step-by-step manual verification guide with complete local commands for:

A completed task result surviving an abrupt VM shutdown.
A task left running without a result being classified as uncertain after restart.
A running invocation checkpoint being restored by a new agent process.
A parallel invocation reusing a completed branch and executing a not-started branch after an abrupt VM shutdown.

These manual integration checks complement the EUnit suite; run both against the exact commit intended for release.

Execution lifecycle

The node validates an execution request, edge tasks, and timeout.
A reducer is started and activated when result consolidation is required.
The node starts and monitors edge processes.
Each edge runs its task and sends its result to the reducer and node.
The reducer consolidates the edge results and reports the final result.
The node updates execution status, notifies any configured recipient, and tracks process termination.

Logical edge IDs and execution IDs correlate messages. Stale notifications must not overwrite terminal execution state or trigger duplicate notifications.

Telemetry

Hayata uses Telemetry to expose lifecycle events from agents, nodes, edges, and reducers. Applications can attach handlers to collect metrics, trace execution activity, or integrate with monitoring systems.

Component	Event	Meaning
Agent	`[hayata, agent, invocation, start]`	An invocation starts.
Agent	`[hayata, agent, invocation, stop]`	An invocation completes or fails normally.
Agent	`[hayata, agent, invocation, exception]`	An invocation encounters a caught policy exception.
Node	`[hayata, node, execution, start]`	A node execution starts.
Node	`[hayata, node, execution, stop]`	A node execution reaches a terminal state, such as completion or cancellation.
Node	`[hayata, node, execution, exception]`	A node execution ends with a reducer recovery failure.
Edge	`[hayata, edge, start]`	An edge task starts.
Edge	`[hayata, edge, stop]`	An edge task ends without a caught task exception.
Edge	`[hayata, edge, exception]`	An edge task raises a caught exception.
Reducer	`[hayata, reducer, start]`	A reducer is activated.
Reducer	`[hayata, reducer, stop]`	A reducer completes or times out.

A stop event indicates a terminal outcome, not necessarily success. Where present, use the status metadata to distinguish completed, failed, cancelled, and timed-out executions.

Start events include system_time. Terminal events include duration, calculated with erlang:monotonic_time/0. Both use Erlang native time units; convert durations when exporting to systems that expect milliseconds.

Common metadata includes agent_id, node_id, execution_id, edge_id, status, and, for caught exceptions, class. Reducer events may also include expected_edges, result_count, error_count, and pending_edges. Available fields vary by event.

Hayata's telemetry metadata avoids including full results, exception reasons, or stacktraces. Applications should also review their own handlers and exporters to avoid exposing sensitive inputs, state, prompts, or outputs.

Attaching a handler

-module(my_hayata_telemetry).

-export([attach/0, detach/0, handle_event/4]).

-define(HANDLER_ID, {?MODULE, agent_invocations}).

attach() ->
    telemetry:attach_many(
        ?HANDLER_ID,
        [
            [hayata, agent, invocation, start],
            [hayata, agent, invocation, stop],
            [hayata, agent, invocation, exception]
        ],
        fun ?MODULE:handle_event/4,
        #{}
    ).

detach() ->
    telemetry:detach(?HANDLER_ID).

handle_event(Event, Measurements, Metadata, _Config) ->
    logger:info(
        "Hayata telemetry event",
        #{
            event => Event,
            measurements => Measurements,
            metadata => Metadata
        }
    ).

Call my_hayata_telemetry:attach/0 during application startup and my_hayata_telemetry:detach/0 when the handler is no longer needed. Keep handlers lightweight; forward expensive processing to a dedicated process or metrics backend.

Fault handling and process supervision

The edge watcher monitors the node independently. If the node dies, including through an untrappable kill, the watcher terminates its edge even though the node's terminate/2 callback cannot run. Node processes are supervised and restarted after abnormal termination.

Known limitation: edge task functions execute synchronously inside the edge GenServer callback. A blocked task prevents that GenServer from processing messages until the callback returns. The watcher can terminate the edge when the node disappears, but cancellation semantics for blocked tasks and their side effects remain an area for further review.

Design principles

Use OTP processes and monitoring for concurrency and failure detection.
Keep process responsibilities and state ownership explicit.
Treat logical IDs separately from process PIDs.
Keep cancellation and recovery scoped to the affected execution.
Persist task outcomes before treating them as completed.
Reject uncertain execution states rather than silently repeating potentially non-idempotent work.
Validate behavior incrementally with EUnit and explicit VM-restart checks.

License

Apache-2.0. See LICENSE.md for details.