5.1 What Sentinel Is and When You Need It
What Sentinel Is and When You Need It
By the end of this subsection, you will understand the difference between Sentinel and Defender XDR, know when an organization needs Sentinel, and be able to explain the two-layer architecture (workspace + Sentinel).
Sentinel vs Defender XDR — complementary, not competing
This is the most common confusion in the Microsoft security stack. Defender XDR and Sentinel are not competing products. They do different things and work together.
Defender XDR correlates alerts from Microsoft Defender products (Endpoint, Office 365, Identity, Cloud Apps) into unified incidents. It works automatically, requires no configuration beyond licensing, and handles the majority of incident detection and response for Microsoft-only environments. You used it in Module 3.
Microsoft Sentinel is a cloud-native SIEM (Security Information and Event Management) and SOAR (Security Orchestration, Automation, and Response). It ingests data from any source — Microsoft, third-party, custom — stores it in a Log Analytics workspace, and provides custom detection rules, hunting, automation, and reporting.
| Capability | Defender XDR | Sentinel |
|---|---|---|
| Data sources | Microsoft Defender products only | Any source (Microsoft + third-party + custom) |
| Detection rules | Microsoft-managed (you tune, not create) | You build custom rules from scratch |
| Automation | Attack disruption (high-confidence only) | Playbooks and automation rules (any trigger) |
| Hunting | Advanced Hunting (Microsoft tables) | Hunting across all ingested data |
| Cost | Included in product licenses | Pay-per-GB ingestion + Sentinel pricing |
| Configuration | Minimal (license and go) | Significant (workspace, connectors, rules, retention) |
Every organization with Microsoft Defender products already has Defender XDR. The question is whether you also deploy Sentinel. If your environment is 100% Microsoft with no compliance requirements for centralized logging, Defender XDR alone may be sufficient. If you have third-party firewalls, SaaS apps, custom applications, or compliance mandates for centralized SIEM — you need Sentinel.
When to deploy Sentinel
Deploy Sentinel when any of these conditions are true:
You have non-Microsoft data sources. Palo Alto firewalls, CrowdStrike endpoints, Okta identity, AWS CloudTrail, Cisco switches, Linux servers, custom applications. Defender XDR cannot ingest any of these. Sentinel can ingest all of them.
You need custom detection rules. Defender XDR’s detections are Microsoft-managed. You can tune severity and suppress false positives, but you cannot write a detection rule from scratch. Sentinel analytics rules are fully custom KQL queries on a schedule — you control every aspect.
Compliance requires centralized logging. Regulations like SOC 2, ISO 27001, HIPAA, and PCI-DSS often require centralized log collection, retention, and monitoring. Sentinel is the Microsoft platform for this.
You need automated response beyond attack disruption. Defender XDR’s attack disruption handles specific high-confidence scenarios. Sentinel playbooks (Logic Apps) can automate any workflow: enrich alerts with threat intelligence, create tickets in ServiceNow, post to Slack, block IPs on a firewall, isolate devices, revoke sessions — triggered by any condition you define.
You manage multiple tenants or complex environments. Managed security providers (MSSPs) and large enterprises with multiple M365 tenants use Sentinel to centralize security operations across all of them.
Try it yourself
Yes — three independent reasons:
1. Third-party data: Palo Alto firewalls and Cisco Meraki generate critical network telemetry that Defender XDR cannot ingest. Without Sentinel, firewall logs exist in isolation with no correlation to identity or endpoint data.
2. SOC 2 compliance: Requires centralized log collection with defined retention and monitoring. Sentinel provides this with configurable retention and automated detection.
3. On-premises server: Windows Server event logs need centralized collection. The Azure Monitor Agent sends them to the Log Analytics workspace where Sentinel can detect lateral movement, privilege escalation, and other server-side threats.
The two-layer architecture
Sentinel is not a standalone product. It is a solution layer that sits on top of a Log Analytics workspace.
Layer 1: Log Analytics workspace. The data store. All ingested data lands here as tables. KQL queries run against these tables. You configure retention, access control, and log tiers at this layer. Without the workspace, there is no data.
Layer 2: Microsoft Sentinel. The security intelligence layer. Analytics rules, incidents, automation, hunting, workbooks, and threat intelligence all run at this layer. Without Sentinel enabled, the workspace is just a database — no detections, no incidents, no automation.
You create the workspace first, then enable Sentinel on it. This is the workflow in subsection 5.3.
Check your understanding
1. An organization uses M365 E5 with Defender for Endpoint and Defender for Office 365. They have no third-party security tools and no compliance requirements for centralized logging. Do they need Sentinel?
2. What is the relationship between a Log Analytics workspace and Sentinel?