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Commit 041ec80

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nblumhardtnblumhardt
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Merge pull request #47 from trevhunter/fix-timestamp
 Fixes #46 - adds pseudo high res timestamp generator.
2 parents 4283ea3 + 054e90e commit 041ec80

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+218
-1
lines changed

4 files changed

+218
-1
lines changed

‎src/InfluxDB.Collector/MetricsCollector.cs

Lines changed: 3 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -7,6 +7,8 @@ namespace InfluxDB.Collector
77
{
88
public abstract class MetricsCollector : IPointEmitter, IDisposable
99
{
10+
readonly Util.ITimestampSource _timestampSource = new Util.PseudoHighResTimestampSource();
11+
1012
public void Increment(string measurement, long count = 1, IReadOnlyDictionary<string, string> tags = null)
1113
{
1214
Write(measurement, new Dictionary<string, object> { { "count", count } }, tags);
@@ -38,7 +40,7 @@ public void Write(string measurement, IReadOnlyDictionary<string, object> fields
3840
{
3941
try
4042
{
41-
var point = new PointData(measurement, fields, tags, timestamp ?? DateTime.UtcNow);
43+
var point = new PointData(measurement, fields, tags, timestamp ?? _timestampSource.GetUtcNow());
4244
Emit(new[] { point });
4345
}
4446
catch (Exception ex)

‎src/InfluxDB.Collector/Util/ITimestampSource.cs

Lines changed: 12 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,12 @@
1+
using System;
2+
3+
namespace InfluxDB.Collector.Util
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{
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/// <summary>
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/// Supplier of timestamps for metrics
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/// </summary>
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interface ITimestampSource
9+
{
10+
DateTime GetUtcNow();
11+
}
12+
}

‎src/InfluxDB.Collector/Util/PseudoHighResTimestampSource.cs

Lines changed: 50 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,50 @@
1+
using System;
2+
3+
namespace InfluxDB.Collector.Util
4+
{
5+
/// <summary>
6+
/// Implements <see cref="ITimestampSource"/>
7+
/// in a way that combines the low-ish resolution DateTime.UtcNow
8+
/// with a sequence number added to the ticks to provide
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/// pseudo-tick precision timestamps
10+
/// </summary>
11+
/// <remarks>
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/// See https://github.com/influxdata/influxdb-csharp/issues/46 for why this is necessary.
13+
/// Long story short:
14+
/// a) InfluxDB has a "LastWriteWins" policy for points that have the same timestamp and tags.
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/// b) The normal <see cref="System.DateTime.UtcNow"/> only supplies timestamps that change not as often as you think.
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/// c) In a web server, it's entirely possible for more than one thread to get the same UtcNow value
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///
18+
/// As a remediation for this, we infuse DateTime.UtcNow with a sequence number until it ticks over.
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/// </remarks>
20+
class PseudoHighResTimestampSource : ITimestampSource
21+
{
22+
private long _lastUtcNowTicks = 0;
23+
private long _sequence = 0;
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private readonly object lockObj = new object();
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public DateTime GetUtcNow()
27+
{
28+
DateTime utcNow = DateTime.UtcNow;
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30+
lock (lockObj)
31+
{
32+
if (utcNow.Ticks == _lastUtcNowTicks)
33+
{
34+
// UtcNow hasn't rolled over yet, so
35+
// add a sequence number to it
36+
_sequence++;
37+
long pseudoTicks = utcNow.Ticks + _sequence;
38+
return new DateTime(pseudoTicks, DateTimeKind.Utc);
39+
}
40+
else
41+
{
42+
// Reset as UtcNow has rolled over
43+
_sequence = 0;
44+
_lastUtcNowTicks = utcNow.Ticks;
45+
return utcNow;
46+
}
47+
}
48+
}
49+
}
50+
}

‎test/InfluxDB.LineProtocol.Tests/Collector/PseudoHighResTimestampSourceTests.cs

Lines changed: 153 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,153 @@
1+
using System;
2+
using Xunit;
3+
using System.Threading.Tasks;
4+
using Xunit.Abstractions;
5+
using InfluxDB.Collector.Util;
6+
7+
namespace InfluxDB.LineProtocol.Tests.Collector.Util
8+
{
9+
public class PseudoHighResTimeStampSourceTests
10+
{
11+
private readonly ITestOutputHelper output;
12+
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public PseudoHighResTimeStampSourceTests(ITestOutputHelper output)
14+
{
15+
this.output = output;
16+
}
17+
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[Fact]
19+
public void CanSupplyHighResTimestamps()
20+
{
21+
const int iterations = 100000;
22+
23+
// Even if we call inside a very tight loop,
24+
// we should get better (pseudo) resolution than just DateTime.UtcNow
25+
long dateTimeCollisions = 0;
26+
DateTime previousDateTime = DateTime.UtcNow;
27+
for (int i = 0; i < iterations; i++)
28+
{
29+
// Attempt with date time directly
30+
DateTime current = DateTime.UtcNow;
31+
32+
if (previousDateTime == current)
33+
{
34+
dateTimeCollisions++;
35+
}
36+
previousDateTime = current;
37+
}
38+
39+
if (0 == dateTimeCollisions)
40+
{
41+
// Warn because we do expect that datetime.now has collisions in such a tight loop
42+
output.WriteLine("Warning! Expected DateTime.UtcNow to have some collisions, but seemed to be high resolution already.");
43+
}
44+
45+
// Now attempt to use our pseudo high precision provider that includes a sequence number to ensure that it
46+
// never collides
47+
ITimestampSource target = new PseudoHighResTimestampSource();
48+
long highResTotalCollisions = 0;
49+
previousDateTime = target.GetUtcNow();
50+
for (int i = 0; i < iterations; i++)
51+
{
52+
// Attempt with date time directly
53+
DateTime current = target.GetUtcNow();
54+
55+
if (previousDateTime == current)
56+
{
57+
highResTotalCollisions++;
58+
}
59+
previousDateTime = current;
60+
}
61+
62+
Assert.Equal(0, highResTotalCollisions);
63+
output.WriteLine($"No collisions detected with high resolution source, compared to {dateTimeCollisions} for DateTime.UtcNow.");
64+
}
65+
66+
[Fact]
67+
public void CanSupplyHighResTimestampsInParallel()
68+
{
69+
const int iterations = 100000;
70+
71+
// Even if we call inside a very tight loop, we should get better (pseudo) resolution than just DateTime.UtcNow
72+
int dateTimeCollisions = 0;
73+
DateTime previousDateTime = DateTime.UtcNow;
74+
Parallel.For(0, iterations, (i) => // (int i = 0; i < 100000; i++)
75+
{
76+
// Attempt with date time directly
77+
DateTime current = DateTime.UtcNow;
78+
79+
if (previousDateTime == current)
80+
{
81+
System.Threading.Interlocked.Increment(ref dateTimeCollisions);
82+
}
83+
previousDateTime = current;
84+
});
85+
86+
if (0 == dateTimeCollisions)
87+
{
88+
// Warn because we do expect that datetime.now has collisions in such a tight loop
89+
output.WriteLine("Warning! Expected DateTime.UtcNow to have some collisions, but seemed to be high resolution already.");
90+
}
91+
92+
// Now attempt to use our pseudo high precision provider that includes a sequence number to ensure that it
93+
// never collides
94+
ITimestampSource target = new PseudoHighResTimestampSource();
95+
int highResTotalCollisions = 0;
96+
previousDateTime = target.GetUtcNow();
97+
Parallel.For(0, iterations, (i) => // (int i = 0; i < 100000; i++)
98+
{
99+
// Attempt with date time directly
100+
DateTime current = target.GetUtcNow();
101+
102+
if (previousDateTime == current)
103+
{
104+
System.Threading.Interlocked.Increment(ref highResTotalCollisions);
105+
}
106+
previousDateTime = current;
107+
});
108+
109+
Assert.Equal(0, highResTotalCollisions);
110+
output.WriteLine($"No collisions detected with high resolution source, compared to {dateTimeCollisions} for DateTime.UtcNow.");
111+
}
112+
113+
[Fact]
114+
public void WillGiveUtcDateTimeKind()
115+
{
116+
ITimestampSource target = new PseudoHighResTimestampSource();
117+
118+
DateTime result = target.GetUtcNow();
119+
Assert.Equal(DateTimeKind.Utc, result.Kind);
120+
}
121+
122+
[Fact]
123+
public void WillNotDriftTooFarFromUtcNow()
124+
{
125+
ITimestampSource target = new PseudoHighResTimestampSource();
126+
const int MAX_DRIFT_MS = 10;
127+
128+
// Average over 10000 iterations and get the average drift
129+
decimal totalDrift = 0;
130+
const int iterations = 10000;
131+
for (int i = 0; i < iterations; i++)
132+
{
133+
DateTime current = DateTime.UtcNow;
134+
DateTime result = target.GetUtcNow();
135+
136+
totalDrift += Convert.ToDecimal((result - current).TotalMilliseconds);
137+
}
138+
Decimal averageDrift = totalDrift / iterations;
139+
140+
if (averageDrift > MAX_DRIFT_MS)
141+
{
142+
output.WriteLine($"Expected times were more than {MAX_DRIFT_MS}ms apart. Instead they were {averageDrift}ms apart.");
143+
Assert.True(false); // Force fail.
144+
}
145+
else
146+
{
147+
output.WriteLine ($"Total Drift over {iterations} iterations: {totalDrift}ms. Average {averageDrift}ms");
148+
}
149+
}
150+
}
151+
}
152+
153+

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