diff --git a/src/node.zig b/src/node.zig
index c567d67..8d8d022 100644
--- a/src/node.zig
+++ b/src/node.zig
@@ -65,6 +65,7 @@ const Descriptor = struct {
 gpa: Allocator,
 trace: *Trace,
 prng: *std.Random.DefaultPrng,
+id: u32,
 local_time: u64,
 arena: std.heap.ArenaAllocator,
 argv: [][*:0]const u8,
@@ -114,10 +115,11 @@ fn splitCommandArguments(command: []const u8, arena: Allocator) Allocator.Error!
     return result;
 }
 
-pub fn init(self: *Node, real_io: std.Io, trace: *Trace, prng: *std.Random.DefaultPrng, scheduler: *Scheduler, network: *Network, command: []const u8, addresses: []const u32, gpa: Allocator) !void {
+pub fn init(self: *Node, real_io: std.Io, trace: *Trace, prng: *std.Random.DefaultPrng, scheduler: *Scheduler, network: *Network, node_id: u32, command: []const u8, addresses: []const u32, gpa: Allocator) !void {
     self.gpa = gpa;
     self.trace = trace;
     self.prng = prng;
+    self.id = node_id;
     self.local_time = 0;
     self.arena = .init(gpa);
     self.scheduler = scheduler;
diff --git a/src/simulator.zig b/src/simulator.zig
index 199cab9..d5c620f 100644
--- a/src/simulator.zig
+++ b/src/simulator.zig
@@ -33,16 +33,18 @@ network: Network,
 nodes: std.ArrayList(*Node),
 executables: std.ArrayList(ExecutableName),
 real_io: std.Io,
+next_node_id: u32,
 
 pub fn init(self: *Simulator, gpa: Allocator, real_io: std.Io, seed: u64) void {
     self.gpa = gpa;
-    self.trace.init(real_io);
+    self.trace.init(&self.scheduler, real_io);
     self.prng = std.Random.DefaultPrng.init(seed);
     self.scheduler.init(gpa, &self.trace, &self.prng);
     self.network.init(gpa);
     self.nodes = .empty;
     self.executables = .empty;
     self.real_io = real_io;
+    self.next_node_id = 0;
 }
 
 pub fn deinit(self: *Simulator) void {
@@ -72,10 +74,13 @@ pub fn spawn(self: *Simulator, command: []const u8, options: SpawnOptions) Spawn
     const name = extractProgramNameFromCommand(command) orelse return SpawnError.InvalidCommand;
     const entry = self.getExecutableEntryPoint(name) orelse return SpawnError.NoSuchProgram;
 
+    const node_id = self.next_node_id;
+    self.next_node_id += 1;
+
     const node = try self.gpa.create(Node);
     errdefer self.gpa.destroy(node);
 
-    try node.init(self.real_io, &self.trace, &self.prng, &self.scheduler, &self.network, command, options.addresses, self.gpa);
+    try node.init(self.real_io, &self.trace, &self.prng, &self.scheduler, &self.network, node_id, command, options.addresses, self.gpa);
 
     try self.nodes.append(self.gpa, node);
     errdefer _ = self.nodes.swapRemove(self.nodes.items.len-1);
diff --git a/src/trace.zig b/src/trace.zig
index a3fe1e1..b9cd3c7 100644
--- a/src/trace.zig
+++ b/src/trace.zig
@@ -1,9 +1,8 @@
 const std = @import("std");
 const Io = std.Io;
 const Node = @import("node.zig");
-const TaskID = @import("scheduler.zig").TaskID;
-
-const PendingTrace = struct {};
+const Scheduler = @import("scheduler.zig");
+const TaskID = Scheduler.TaskID;
 
 pub const Trace = struct {
 
@@ -15,14 +14,43 @@ pub const Trace = struct {
         waiting_task,
         failed,
         returned,
+
+        pub fn toString(self: TaskState) []const u8 {
+            return switch (self) {
+                .ready         => "ready",
+                .running       => "running",
+                .sleeping      => "sleeping",
+                .waiting_futex => "waiting-futex",
+                .waiting_task  => "waiting-task",
+                .failed        => "failed",
+                .returned      => "returned",
+            };
+        }
+    };
+
+    const Pending = struct {
+        trace_id: u64,
+        local_time: u64,
+        global_time: u64,
+        node_id: u32,
+        task_id: TaskID,
+        disk: bool,
+        op: []const u8,
     };
 
     io: Io,
     file: ?Io.File,
+    scheduler: *Scheduler,
+    task: ?TaskID,
+    node: ?*Node,
+    next_trace_id: u64,
 
-    pub fn init(self: *Trace, io: Io) void {
+    pub fn init(self: *Trace, scheduler: *Scheduler, io: Io) void {
         self.io = io;
         self.file = null;
+        self.scheduler = scheduler;
+        self.task = null;
+        self.node = null;
     }
 
     pub fn deinit(self: *Trace) void {
@@ -38,65 +66,199 @@ pub const Trace = struct {
     pub fn enterTask(self: *Trace, task_id: TaskID, node: *Node) void {
         if (self.file == null) return;
 
-        _ = task_id;
-        _ = node;
+        self.task = task_id;
+        self.node = node;
     }
 
     pub fn leaveTask(self: *Trace) void {
         if (self.file == null) return;
 
+        self.task = null;
+        self.node = null;
     }
 
     pub fn taskSpawned(self: *Trace, task_id: TaskID, node: *Node, parent_id: ?TaskID) void {
         if (self.file == null) return;
 
-        _ = task_id;
-        _ = node;
-        _ = parent_id;
+        const global_time = self.scheduler.current_time;
+        const local_time = node.local_time;
+
+        var line_buffer: [256]u8 = undefined;
+        const prefix = std.fmt.bufPrint(&line_buffer,
+            \\{{"time":{},"global_time":{},"event":"task_spawned","node":{},"task":{}
+        , .{ local_time, global_time, node.id, task_id }) catch return;
+        self.write(prefix);
+        if (parent_id) |it| {
+            const parent = std.fmt.bufPrint(&line_buffer, ",\"parent\":{}", .{it}) catch return;
+            self.write(parent);
+        }
+        self.write("}\n");
     }
 
     pub fn taskRemoved(self: *Trace, task_id: TaskID, node: *Node) void {
         if (self.file == null) return;
 
-        _ = task_id;
-        _ = node;
+        const global_time = self.scheduler.current_time;
+        const local_time  = node.local_time;
+
+        var line_buffer: [256]u8 = undefined;
+        const line = std.fmt.bufPrint(&line_buffer,
+            \\{{"time":{},"global_time":{},"event":"task_removed","node":{},"task":{}}}
+            \\
+        , .{ local_time, global_time, node.id, task_id }) catch return;
+        self.write(line);
     }
 
     pub fn taskState(self: *Trace, task_id: TaskID, node: *Node, state: Trace.TaskState, reason: []const u8) void {
         if (self.file == null) return;
 
-        _ = task_id;
-        _ = node;
-        _ = state;
-        _ = reason;
+        const global_time = self.scheduler.current_time;
+        const local_time = node.local_time;
+
+        var line_buffer: [256]u8 = undefined;
+        const prefix = std.fmt.bufPrint(&line_buffer,
+            \\{{"time":{},"global_time":{},"event":"state","node":{},"task":{},"state":
+        , .{ local_time, global_time, node.id, task_id }) catch return;
+        self.write(prefix);
+        self.writeJsonString(state.toString());
+        self.write(",\"reason\":");
+        self.writeJsonString(reason);
+        self.write("}\n");
     }
 
     pub fn timeAdvanced(self: *Trace, from: u64, to: u64) void {
         if (self.file == null) return;
 
-        _ = from;
-        _ = to;
+        var line_buffer: [256]u8 = undefined;
+        const line = std.fmt.bufPrint(&line_buffer,
+            \\{{"time":{},"global_time":{},"event":"time_advanced","from":{},"to":{}}}
+            \\
+        , .{ to, to, from, to }) catch return;
+        self.write(line);
+
     }
 
-    pub fn beginIO(self: *Trace, disk: bool, source: std.builtin.SourceLocation) PendingTrace {
-        if (self.file == null) return .{};
+    pub fn beginIO(self: *Trace, disk: bool, source: std.builtin.SourceLocation) Pending {
+        if (self.file == null)
+            return .{
+                .trace_id = 0,
+                .local_time = 0,
+                .global_time = 0,
+                .task_id = 0,
+                .node_id = 0,
+                .disk = false,
+                .op = &.{},
+            };
 
-        _ = disk;
-        _ = source;
-        return .{};
+        std.debug.assert(self.task != null);
+        std.debug.assert(self.node != null);
+        const node = self.node.?;
+        const task = self.task.?;
+
+        const global_time = self.scheduler.current_time;
+        const local_time  = node.local_time;
+
+        const trace_id = self.next_trace_id;
+        self.next_trace_id += 1;
+
+        var line_buffer: [256]u8 = undefined;
+        const prefix = std.fmt.bufPrint(&line_buffer,
+            \\{{"time":{},"global_time":{},"event":"io_start","id":{},"node":{},"task":{},"op":
+        , .{ local_time, global_time, trace_id, node.id, task }) catch @panic("TODO");
+        self.write(prefix);
+        self.writeJsonString(source.fn_name);
+        self.write(",\"resource\":");
+        self.writeJsonString(if (disk) "disk" else "io");
+        self.write("}\n");
+
+        return .{
+            .trace_id = trace_id,
+            .local_time = local_time,
+            .global_time = global_time,
+            .node_id = node.id,
+            .task_id = task,
+            .disk = disk,
+            .op = source.fn_name,
+        };
     }
 
-    pub fn failIO(self: *Trace, pending_trace: PendingTrace, e: anyerror) void {
+    pub fn failIO(self: *Trace, pending_trace: Pending, e: anyerror) void {
         if (self.file == null) return;
 
-        _ = pending_trace;
-        _ = e catch {};
+        var result_buffer: [96]u8 = undefined;
+        const result = std.fmt.bufPrint(&result_buffer, "error.{s}", .{@errorName(e)}) catch "error";
+        self.completeIO(pending_trace, result);
     }
 
-    pub fn completeIO(self: *Trace, pending_trace: PendingTrace, result: anytype) void {
+    pub fn completeIO(self: *Trace, pending_trace: Pending, result: anytype) void {
         if (self.file == null) return;
 
-        _ = pending_trace;
-        _ = result;
+        const global_time = self.scheduler.current_time;
+        const node = self.node orelse return;
+        const local_time = node.local_time;
+
+        var line_buffer: [256]u8 = undefined;
+        const prefix = std.fmt.bufPrint(&line_buffer,
+            \\{{"time":{},"global_time":{},"event":"io_complete","id":{},"node":{},"task":{},"op":
+            , .{ local_time, global_time, pending_trace.trace_id, pending_trace.node_id, pending_trace.task_id }) catch return;
+        self.write(prefix);
+        self.writeJsonString(pending_trace.op);
+        self.write(",\"resource\":");
+        self.writeJsonString(if (pending_trace.disk) "disk" else "io");
+        const suffix = std.fmt.bufPrint(&line_buffer,
+            \\,"start":{},"end":{},"result":
+            , .{ pending_trace.local_time, local_time }) catch return;
+        self.write(suffix);
+        var result_buffer: [96]u8 = undefined;
+        self.writeJsonString(resultText(&result_buffer, result));
+        self.write("}\n");
+
+        if (pending_trace.disk) {
+            var disk_line_buffer: [256]u8 = undefined;
+            const disk_prefix = std.fmt.bufPrint(&disk_line_buffer,
+                \\{{"time":{},"global_time":{},"event":"disk","node":{},"op":
+                , .{ pending_trace.local_time, global_time, pending_trace.node_id }) catch return;
+            self.write(disk_prefix);
+            self.writeJsonString(pending_trace.op);
+            const middle = std.fmt.bufPrint(&disk_line_buffer,
+                \\,"start":{},"end":{},"detail":
+                , .{ pending_trace.local_time, local_time }) catch return;
+            self.write(middle);
+            self.writeJsonString(resultText(&result_buffer, result));
+            self.write("}\n");
+        }
+    }
+
+    fn writeJsonString(self: *Trace, text: []const u8) void {
+        self.write("\"");
+        for (text) |byte| {
+            switch (byte) {
+                '"' => self.write("\\\""),
+                '\\' => self.write("\\\\"),
+                '\n' => self.write("\\n"),
+                '\r' => self.write("\\r"),
+                '\t' => self.write("\\t"),
+                else => self.write((&byte)[0..1]),
+            }
+        }
+        self.write("\"");
+    }
+
+    fn write(self: *Trace, bytes: []const u8) void {
+        const file = self.file orelse return;
+        file.writeStreamingAll(self.io, bytes) catch {};
     }
 };
+
+fn resultText(buffer: []u8, result: anytype) []const u8 {
+    const T = @TypeOf(result);
+    return switch (@typeInfo(T)) {
+        .void => "ok",
+        .@"struct" => "ok",
+        .pointer => |pointer| if (pointer.size == .slice and pointer.child == u8)
+            result
+        else
+            std.fmt.bufPrint(buffer, "{any}", .{result}) catch "result",
+        else => std.fmt.bufPrint(buffer, "{}", .{result}) catch "result",
+    };
+}