CMDB - Understanding Identification and Reconciliation Rules and Conflicts within these rules

anubhavritolia
Giga Sage

5 hours ago

Big Picture (one‑liner)

👉Identification & Reconciliation rules answer two different questions:

 

Question
Answered by
“Is this CI already existing or is it new?”
Identification Rules
“If multiple sources update the same CI, whose data should I trust?”
Reconciliation Rules

 

1️⃣Identification Rules (IR)

 

🔹What are Identification Rules?

Identification Rules decide HOW ServiceNow recognizes a CI.

In simple words:

This rule tells ServiceNow how to uniquely identify a CI record.

🧠 Think of it like Aadhaar / PAN Card

  • Aadhaar number uniquely identifies you
  • PAN card also identifies you
  • Same person → not two people

Similarly:

  • Serial Number
  • IP Address
  • Host name

These help identify one unique CI

✅What happens without Identification Rules?

If identification is weak:

  • Same server gets created multiple times
  • CMDB becomes dirty
  • Reports & dependency maps break

🔧Where are Identification Rules used?

Mostly during:

  • Discovery
  • Service Mapping
  • Import Sets
  • Integrations (SCCM, Azure, AWS, etc.)

📘Simple Example – Server CI

CI Class: cmdb_ci_server

Identification Attribute
Why
serial_number
Hardware-level unique
(OR) name + IP address
Logical identification

 

Rule:

IF serial_number matches → SAME CI
ELSE create NEW CI

✅Result:

  • Discovery runs daily
  • Same server updates
  • No duplicates created

📘Example – Network Device

Attribute Used for Identification
MAC Address Primary
IP AddressCan change

 

👉Identification Rule:

Use MAC Address to identify network device

🔑Key Takeaway for Identification Rules

✅Identification decides CREATE vs UPDATE

“Is this CI already there or not?”

2️⃣Reconciliation Rules (RR)

 

🔹What are Reconciliation Rules?

Reconciliation rules decide WHO wins when multiple sources update the same CI attribute.

In simple words:

This rule decides which source’s data is trusted more.

🧠 Think of it like conflicting information

Suppose:

  • Your passport says DOB = 1 Jan 1990
  • Your driving license says DOB = 2 Jan 1990

❓Which one should be trusted?

👉You decide a priority Same concept here.

✅Why Reconciliation Rules are needed

In CMDB:

  • Multiple tools update the same CI
    • Discovery
    • SCCM
    • Cloud integrations
    • Manual updates

Without rules:

  • Data keeps flipping
  • CMDB becomes unreliable

🔧Where are Reconciliation Rules applied?

  • Attribute level
  • CI level
  • Source-based priority

📘Simple Example – Server OS Version

CI: Server-01

SourceOS Version
DiscoveryWindows 2019
SCCMWindows 2016
Manual UpdateWindows 2022

 

✅Reconciliation Rule (Priority-based)

Discovery → Priority 1
SCCM → Priority 2
Manual → Priority 3

✅Final value saved:

OS Version = Windows 2019
(from Discovery)

📘Example – IP Address Conflict


SourceIP
Discovery10.10.1.5
Import Set10.10.1.7

 

Rule:

Discovery has higher priority → wins

✅CMDB keeps 10.10.1.5

🔑Key Takeaway for Reconciliation Rules

✅Reconciliation decides SOURCE PRIORITY

“If multiple sources update the same CI, whose data should I trust?”

3️⃣Identification vs Reconciliation – Side-by-Side


AspectIdentification RulesReconciliation Rules
Main QuestionIs this CI new or existing?Which source wins?
Used WhenCI creation / matchingCI update conflicts
Works OnCI identityCI attributes
PreventsDuplicate CIsData overwrites
ExampleSerial NumberDiscovery vs SCCM

4️⃣End-to-End Example (Very Important)

🔄Discovery Flow

  1. Discovery runs
  2. Identification Rule
    • Serial Number matches → ✅ update existing CI
  3. Reconciliation Rule
    • Discovery beats SCCM → ✅ Discovery values kept
  4. ✅CMDB stays clean and trusted

🧠 One-line Memory Trick (for exams & interviews)

Identification = “Who are you?”
Reconciliation = “Whose data do I believe?”

👉Identification Rules are evaluated first
👉Reconciliation Rules apply only after a CI is identified
👉If conflicts occur, ServiceNow follows a strict order of precedence, not “random behavior”
 

Lets Discuss Conflicts in details below:

 

1️⃣Conflict Type 1: Identification Rules Conflict

 

❓What does this mean?

More than one CI matches the same incoming data during identification.

📘Example

Incoming data from Discovery:

Serial Number = ABC123
IP Address = 10.10.1.5

Existing CIs:

CISerial NumberIP
CI‑1ABC12310.10.1.4
CI‑2ABC12410.10.1.5

 

👉One rule matches CI‑1
👉Another rule matches CI‑2

❗Conflict detected

✅What ServiceNow Does

ServiceNow will NOT guess

It will:

  • ❌Fail identification
  • ❌NOT update any CI
  • ✅Log an IRE (Identification Engine) error
  • ✅Mark record as “Ambiguous match”

📍Result:

CI NOT updated
No duplicate created
Admin must fix identification rules or CMDB data

🔑Key Rule

If Identification is ambiguous, CMDB refuses to process

This is by design to protect data integrity.

2️⃣Conflict Type 2: Multiple Identification Rules Apply

 

❓What does this mean?

More than one valid identification rule exists for the same CI class.

✅How ServiceNow resolves it

Identification Rules are processed in this order:

  1. Class hierarchy

    • More specific class wins
      (Server > Computer > CI)

  2. Rule priority (Order field)

    • Lower order = higher priority

  3. Attribute strength

    • Strong identifiers (Serial Number)
    • Medium (Name + IP)
    • Weak (Name only)

✅The first successful rule wins

📘Example

Rules for cmdb_ci_server:


RuleAttributesOrder
Rule‑1Serial Number100
Rule‑2Name + IP200

✅If Serial Number matches → Rule‑1 used
✅Rule‑2 is ignored

3️⃣Conflict Type 3: Reconciliation Rules Conflict

 

❓What does this mean?

Two or more data sources try to update the same attribute.

📘Example

CI Attribute: Operating System

Source Value Priority
SourceValuePriority
DiscoveryWindows 20191
SCCMWindows 20222
ManualWindows 20163

✅What ServiceNow Does

ServiceNow uses Source Priority (lower number = higher priority)

✅Final value:

Windows 2019
(Source = Discovery)

🔑Important Rule

Only ONE source wins per attribute

There is no merging logic unless custom logic is built.

4️⃣Conflict Between Identification & Reconciliation

 

This is a very common misunderstanding.

❓Does reconciliation apply if identification fails?

❌NO

✅Actual Processing Order

1️⃣ Identification
   ├─ If CI is NOT uniquely identified → STOP
2️⃣ Reconciliation
   ├─ Happens ONLY if identification succeeds
3️⃣ Update CI

👉If identification fails:

  • ❌No reconciliation
  • ❌No update
  • ❌No create

5️⃣Conflict Type 4: Same Priority Sources

 

📘Example

Two sources both have priority 1:

  • Discovery
  • Cloud Integration

✅What ServiceNow Does

ServiceNow uses:

  • Last Updated timestamp
  • Source precedence order
  • In some cases → non-deterministic result

⚠️This is considered bad design

✅Best Practice

Always:

  • Avoid equal priorities
  • Clearly define single system of record per attribute

6️⃣How You Detect These Conflicts

 

🔍Tools & Logs

  • Identification Engine Logs
  • cmdb_ire_log
  • Discovery Error Logs
  • CI Audit History

7️⃣Summary Table — What Happens When Rules Conflict


Conflict TypeOutcome❌
Multiple CI match in Identification❌No update, error logged
Multiple Identification rules valid✅Higher priority rule wins
Multiple data sources updating same field✅Higher priority source wins
Identification fails❌Reconciliation never runs
Same source priority⚠️Unpredictable, avoid

🧠 Exam / Interview One‑Liners

✅“Identification conflicts stop CMDB updates to prevent corruption.”
✅“Reconciliation applies only after a CI is uniquely identified.”
✅“Source priority decides reconciliation conflicts.”

 

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Also feel free to provide your comments/feedback on this Article.

 

Regards,

Anubhav Ritolia

Community Rising Star 2023, 2024

Organizer of Pune Developer Meetup

Technical Architect at LTIMindtree

LinkedIn Profile

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