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Problem Diagnosis
• Distributed Problem Diagnosis
• Sherlock
• X-trace
Troubleshooting Networked
Systems
• Hard to develop, debug, deploy, troubleshoot
• No standard way to integrate debugging,
monitoring, diagnostics
Status quo: device centric
Load
Balancer
Web 1
Firewall
...
...
28 03:55:38 PM fire...
28 03:55:38 PM fire...
28 03:55:38 PM fire...
28 03:55:38 PM fire...
28 03:55:38 PM fire...
28 03:55:38 PM fire...
28 03:55:38 PM fire...
28 03:55:39 PM fire...
28 03:55:39 PM fire...
28 03:55:39 PM fire...
28 03:55:39 PM fire...
28 03:55:39 PM fire...
28 03:55:39 PM fire...
28 03:55:39 PM fire...
28 03:55:39 PM fire...
...
...
...
[04:03:23 2006] [notice] Dispatch s1...
[04:03:23 2006] [notice] Dispatch s2...
[04:04:18 2006] [notice] Dispatch s3...
[04:07:03 2006] [notice] Dispatch s1...
[04:10:55 2006] [notice] Dispatch s2...
[04:03:24 2006] [notice] Dispatch s3...
[04:04:47 2006] [crit] Server s3 down...
...
...
...
...
72.30.107.159 - - [20/Aug/2006:09:12:58 -0700] "GET /ga
65.54.188.26 - - [20/Aug/2006:09:13:32 -0700] "GET /rob
65.54.188.26 - - [20/Aug/2006:09:13:32 -0700] "GET /rob
65.54.188.26 - - [20/Aug/2006:09:13:32 -0700] "GET /gal
65.54.188.26 - - [20/Aug/2006:09:13:32 -0700] "GET /gal
66.249.72.163 - - [20/Aug/2006:09:15:04 -0700] "GET /ga
66.249.72.163 - - [20/Aug/2006:09:15:07 -0700] "GET /ga
66.249.72.163 - - [20/Aug/2006:09:15:10 -0700] "GET /ro
66.249.72.163 - - [20/Aug/2006:09:15:11 -0700] "GET /ga
...
...
Web 2
...
...
72.30.107.159 - - [20/Aug/2006:09:12:58 -0700] "GET /ga
65.54.188.26 - - [20/Aug/2006:09:13:32 -0700] "GET /rob
65.54.188.26 - - [20/Aug/2006:09:13:32 -0700] "GET /rob
65.54.188.26 - - [20/Aug/2006:09:13:32 -0700] "GET /gal
65.54.188.26 - - [20/Aug/2006:09:13:32 -0700] "GET /gal
66.249.72.163 - - [20/Aug/2006:09:15:04 -0700] "GET /ga
66.249.72.163 - - [20/Aug/2006:09:15:07 -0700] "GET /ga
66.249.72.163 - - [20/Aug/2006:09:15:10 -0700] "GET /ro
66.249.72.163 - - [20/Aug/2006:09:15:11 -0700] "GET /ga
...
...
Database
...
...
LOG:
LOG:
LOG:
LOG:
LOG:
LOG:
LOG:
LOG:
LOG:
LOG:
LOG:
LOG:
LOG:
LOG:
LOG:
LOG:
LOG:
...
...
statement: select oid...
statement: SELECT COU...
statement: SELECT g2_...
statement: select oid...
statement: SELECT COU...
statement: SELECT g2_...
statement: select oid...
statement: SELECT COU...
statement: SELECT g2_...
statement: select oid...
statement: select oid...
statement: SELECT COU...
statement: SELECT g2_...
statement: select oid...
statement: SELECT COU...
statement: SELECT g2_...
statement: select oid...
Status quo: device centric
• Determining paths:
– Join logs on time and ad-hoc identifiers
• Relies on
– well synchronized clocks
– extensive application knowledge
• Requires all operations logged to guarantee
complete paths
Examples
DNS Server
User
Web Server
Proxy
5
Examples
DNS Server
User
Web Server
Proxy
6
Examples
DNS Server
User
Web Server
Proxy
7
Examples
DNS Server
User
Web Server
Proxy
8
Approaches to Diagnosis
• Passively learn the relationships
– Infer problems as deviations from the norm
• Actively Instrument the stack to learn
relationships
– Infer problems as deviations from the norm
Sherlock – Diagnosing Problems
in the Enterprise
Srikanth Kandula
Well-Managed Enterprises Still Unreliable
Response time of a Web server (ms)
.1
.08
Fraction Of
Requests .06
85%
Normal
10% Troubled
0.7% Down
.04
.02
0
10
100
1000
10000
10% responses take up to 10x longer than normal
How do we manage evolving enterprise networks?
Sherlock
Instead of looking at the nitty-gritty of individual
components, use an end-to-end approach that
focuses on user problems
Challenges for the End-to-End Approach
• Don’t know what user’s
performance depends on
Challenges for the End-to-End Approach
E.g., Web Connection
SQL
Backend
Auth.
Server
DNS
Web
Server
• Don’t know what user’s
performance depends on
– Dependencies are distributed
– Dependencies are non-deterministic
• Don’t know which dependency is
causing the problem
– Server CPU 70%, link dropped 10
packets, but which affected user?
Client
Sherlock’s Contributions
• Passively infers dependencies from logs
• Builds a unified dependency graph incorporating
network, server and application dependencies
• Diagnoses user problems in the enterprise
• Deployed in a part of the Microsoft Enterprise
Sherlock’s Architecture
Sherlock’s Architecture
Network Dependency Graph
+
User Observations
Servers
Inference
Engine
Clients
=
List Troubled
Sherlock works for various client-server
applications
Web2
30ms
Web1
File1
1000ms Timeout
Components
DNS
Video Server
Data Store
How do you automatically learn such distributed
dependencies?
Strawman: Instrument all applications and libraries
 Not Practical
Sherlock exploits timing info
My Client
talks to B
My Client
talks to C
Time
t
If talks to B, whenever talks to C  Dependent Connections
Strawman: Instrument all applications and libraries
 Not Practical
Sherlock exploits timing info
B
B
B
B
C
BB
B
Time
t
False Dependence
If talks to B, whenever talks to C  Dependent Connections
Strawman: Instrument all applications and libraries
 Not Practical
Sherlock exploits timing info
B
C
B
Time
t
Inter-access time
Dependent iff t << Inter-access time
If talks to B, whenever talks to C  Dependent Connections
As long as this occurs with probability higher than chance
Sherlock’s Algorithm to Infer Dependencies
 Infer dependent connections from timing
Video
Store
DNS
Dependency
Graph
Sherlock’s Algorithm to Infer Dependencies
 Infer dependent connections from timing
 Infer topology from Traceroutes & configurations
Video
Store
DNS
Bill’s Client
Video
Store
DNS
Dependency
Graph
Video Store
Bill DNS
Bill Video
Bill Watches Video
• Works with legacy applications
• Adapts to changing conditions
But hard dependencies are not enough…
But hard dependencies are not enough…
DNS
Video
Bill’s Client
Store
Video Store
Bill DNS
Bill Video
p1
p1=10%
p3
p2
p2=100%
Bill watches Video
If Bill caches server’s IP  DNS down but Bill gets video
 Need Probabilities
Sherlock uses the frequency with which a
dependence occurs in logs as its edge probability
How do we use the dependency
graph to diagnose user problems?
Diagnosing User Problems
DNS
Bill’s Client
Video
Store
Video Store
Bill DNS
Bill Video
Bill Watches Video
Which components caused the problem?
Need to disambiguate!!
Diagnosing User Problems
Sales
DNS
Bill’s Client
Video
Store
Video2
Video Store Video2 Store
Bill Sales
Bill Sees Sales
Bill DNS
Bill Video
Bill Watches Video
components
the problem?
•Which
Disambiguate
bycaused
correlating
Use– correlation
disambiguate!!
Across logstofrom
same client
– Across clients
• Prefer simpler explanations
Paul Video2
Paul Watches Video2
Will Correlation Scale?
Will Correlation Scale?
Corporate Core
Building
Network
Microsoft Internal Network
•
•
•
•
O(100,000) client desktops
O(10,000) servers
O(10,000) apps/services
O(10,000) network devices
Campus Core
Data
Center
Dependency
Graph is Huge
Will Correlation Scale?
Can we evaluate all combinations of component failures?
The number of fault combinations is exponential!
Impossible to compute!
Scalable Algorithm to Correlate
Only a few faults happen
concurrently
But how many is few?
Evaluate enough to cover
99.9% of faults
For MS network, at most 2
concurrent faults  99.9%
accurate
Exponential  Polynomial
Scalable Algorithm to Correlate
Only a few faults happen
concurrently
But how many is few?
Evaluate enough to cover
99.9% of faults
For MS network, at most 2
concurrent faults  99.9%
accurate
Exponential  Polynomial
Only few nodes change state
Scalable Algorithm to Correlate
Only a few faults happen
concurrently
Only few nodes change state
But how many is few?
Evaluate enough to cover
99.9% of faults
For MS network, at most 2
concurrent faults  99.9%
accurate
Exponential  Polynomial
Re-evaluate only if an
ancestor changes state
Reduces the cost of
evaluating a case by 30x-70x
Results
Experimental Setup
• Evaluated on the Microsoft enterprise network
• Monitored 23 clients, 40 production servers for 3 weeks
– Clients are at MSR Redmond
– Extra host on server’s Ethernet logs packets
• Busy, operational network
– Main Intranet Web site and software distribution file server
– Load-balancing front-ends
– Many paths to the data-center
What Do Web Dependencies in the MS
Enterprise Look Like?
What Do Web Dependencies in the MS
Enterprise Look Like?
Auth.
Server
Client Accesses Portal
What Do Web Dependencies in the MS
Enterprise Look Like?
Auth.
Server
Client Accesses Portal
What Do Web Dependencies in the MS
Enterprise Look Like?
Auth.
Server
Client Accesses Portal
Client Accesses Sales
Sherlock discovers complex dependencies of real apps.
What Do File-Server Dependencies
Look Like?
Backend
Server 1
Backend
Server 2
8%
File Server
100%
Auth.
Server
WINS
DNS
Proxy
10%
6%
5%
2%
Backend
Server 3
5%
1%
.3%
Backend
Server 4
Client Accesses Software Distribution Server
Sherlock works for many client-server applications
Sherlock Identifies Causes of Poor Performance
Component Index
Dependency Graph: 2565 nodes; 358 components that can fail
Time (days)
87% of problems localized to 16 components
Sherlock Identifies Causes of Poor Performance
Component Index
Inference Graph: 2565 nodes; 358 components that can fail
Time (days)
Corroborated the three significant faults
Sherlock Goes Beyond Traditional Tools
• SNMP-reported utilization on a link flagged by Sherlock
• Problems coincide with spikes
Sherlock identifies the troubled link but SNMP cannot!
X-Trace
• X-Trace records events in a distributed
execution and their causal relationship
• Events are grouped into tasks
– Well defined starting event and all that is
causally related
• Each event generates a report, binding it to
one or more preceding events
• Captures full happens-before relation
X-Trace Output
HTTP
Client
HTTP
Proxy
TCP 1
Start
IP
TCP 1
End
IP
Router
IP
HTTP
Server
TCP 2
Start
IP
TCP 2
End
IP
Router
• Task graph capturing task execution
– Nodes: events across layers, devices
– Edges: causal relations between events
IP
Router
IP
Basic Mechanism
a
g
[T, a]
HTTP
Client
b
TCP 1
End
d
IP
TCP 2
Start
i
e
IP
Router
HTTP
Server
h
f
TCP 1
Start
c
[T, g]
HTTP
Proxy
[T, a]
n
IP
IP
X-Trace Report
TaskID: T
EventID: g
j
k f
Edge:
from a,
IP
Router
IP
Router
m
TCP 2
End
l
IP
• Each event uniquely identified within a task:
[TaskId, EventId]
• [TaskId, EventId] propagated along execution path
• For each event create and log an X-Trace report
– Enough info to reconstruct the task graph
X-Trace Library API
• Handles propagation within app
• Threads / event-based (e.g., libasync)
• Akin to a logging API:
– Main call is logEvent(message)
• Library takes care of event id creation,
binding, reporting, etc
• Implementations in C++, Java, Ruby,
Javascript
Task Tree
• X-Trace tags all network operations
resulting from a particular task with the
same task identifier
• Task tree is the set of network
operations connected with an initial task
• Task tree could be reconstruct after
collecting trace data with reports
52
An example of the task tree
• A simple HTTP request through a proxy
53
X-Trace Components
• Data
– X-Trace metadata
• Network path
– Task tree
• Report
– Reconstruct task tree
54
Propagation of X-Trace Metadata
• The propagation of X-Trace metadata
through the task tree
55
Propagation of X-Trace Metadata
• The propagation of X-Trace metadata
through the task tree
56
The X Trace metadata
Field
Usage
Flags
Bits that specify which of the three optional components are present
TaskID
An unique integer ID
TreeInfo
ParentID, OpID, EdgeType
Destination
Specify the address that X-Trace report should be sent to
Options
Accommodate future extensions mechanism
57
X-Trace Report Architecture
58
X-Trace Report Architecture
59
X-Trace Report Architecture
60
X-Trace-like in
Google/Bing/Yahoo
• Why?
– Own large portion of the ecosystem
– Use RPC for communication
– Need to understand
• Time for user request
• Resource utilization by request
Sherlock V X-trace
• Overhead V. Accuracy
• Deployment issues
– Invasiveness
– Code modification
Conclusions
• Sherlock passively infers network-wide
dependencies from logs and traceroutes
• It diagnoses faults by correlating user observations
• X-trace actively discovers network-wide dependencies